docs: Add release notes for descriptor wallets

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+Wallet
+------
+
+### Experimental Descriptor Wallets
+
+Please note that Descriptor Wallets are still experimental and not all expected functionality
+is available. Additionally there may be some bugs and current functions may change in the future.
+Bugs and missing functionality can be reported to the [issue tracker](https://github.com/bitcoin/bitcoin/issues).
+
+0.21 introduces a new type of wallet - Descriptor Wallets. Descriptor Wallets store
+scriptPubKey information using descriptors. This is in contrast to the Legacy Wallet
+structure where keys are used to generate scriptPubKeys and addresses. Because of this
+shift to being script based instead of key based, many of the confusing things that Legacy
+Wallets do are not possible with Descriptor Wallets. Descriptor Wallets use a definition
+of "mine" for scripts which is simpler and more intuitive than that used by Legacy Wallets.
+Descriptor Wallets also uses different semantics for watch-only things and imports.
+
+As Descriptor Wallets are a new type of wallet, their introduction does not affect existing wallets.
+Users who already have a Bitcoin Core wallet can continue to use it as they did before without
+any change in behavior. Newly created Legacy Wallets (which is the default type of wallet) will
+behave as they did in previous versions of Bitcoin Core.
+
+The differences between Descriptor Wallets and Legacy Wallets are largely limited to non user facing
+things. They are intended to behave similarly except for the import/export and watchonly functionality
+as described below.
+
+#### Creating Descriptor Wallets
+
+Descriptor Wallets are not created by default. They must be explicitly created using the
+`createwallet` RPC or via the GUI. A `descriptors` option has been added to `createwallet`.
+Setting `descriptors` to `true` will create a Descriptor Wallet instead of a Legacy Wallet.
+
+In the GUI, a checkbox has been added to the Create Wallet Dialog to indicate that a
+Descriptor Wallet should be created.
+
+Without those options being set, a Legacy Wallet will be created instead. Additionally the
+Default Wallet created upon first startup of Bitcoin Core will be a Legacy Wallet.
+
+#### `IsMine` Semantics
+
+`IsMine` refers to the function used to determine whether a script belongs to the wallet.
+This is used to determine whether an output belongs to the wallet. `IsMine` in Legacy Wallets
+returns true if the wallet would be able to sign an input that spends an output with that script.
+Since keys can be involved in a variety of different scripts, this definition for `IsMine` can
+lead to many unexpected scripts being considered part of the wallet.
+
+With Descriptor Wallets, descriptors explicitly specify the set of scripts that are owned by
+the wallet. Since descriptors are deterministic and easily enumerable, users will know exactly
+what scripts the wallet will consider to belong to it. Additionally the implementation of `IsMine`
+in Descriptor Wallets is far simpler than for Legacy Wallets. Notably, in Legacy Wallets, `IsMine`
+allowed for users to take one type of address (e.g. P2PKH), mutate it into another address type
+(e.g. P2WPKH), and the wallet would still detect outputs sending to the new address type
+even without that address being requested from the wallet. Descriptor Wallets does not
+allow for this and will only watch for the addresses that were explicitly requested from the wallet.
+
+These changes to `IsMine` will make it easier to reason about what scripts the wallet will
+actually be watching for in outputs. However for the vast majority of users, this change is
+largely transparent and will not have noticeable effect.
+
+#### Imports and Exports
+
+In Legacy Wallets, raw scripts and keys could be imported to the wallet. Those imported scripts
+and keys are treated separately from the keys generated by the wallet. This complicates the `IsMine`
+logic as it has to distinguish between spendable and watchonly.
+
+Descriptor Wallets handle importing scripts and keys differently. Only complete descriptors can be
+imported. These descriptors are then added to the wallet as if it were a descriptor generated by
+the wallet itself. This simplifies the `IsMine` logic so that it no longer has to distinguish
+between spendable and watchonly. As such, the watchonly model for Descriptor Wallets is also
+different and described in more detail in the next section.
+
+To import into a Descriptor Wallet, a new `importdescriptors` RPC has been added that uses a syntax
+similar to that of `importmulti`.
+
+As Legacy Wallets and Descriptor Wallets use different mechanisms for storing and importing scripts and keys
+the existing import RPCs have been disabled for descriptor wallets.
+New export RPCs for Descriptor Wallets have not yet been added.
+
+The following RPCs are disabled for Descriptor Wallets:
+
+* importprivkey
+* importpubkey
+* importaddress
+* importwallet
+* dumpprivkey
+* dumpwallet
+* importmulti
+* addmultisigaddress
+* sethdseed
+
+#### Watchonly Wallets
+
+A Legacy Wallet contains both private keys and scripts that were being watched.
+Those watched scripts would not contribute to your normal balance. In order to see the watchonly
+balance and to use watchonly things in transactions, an `include_watchonly` option was added
+to many RPCs that would allow users to do that. However it is easy to forget to include this option.
+
+Descriptor Wallets move to a per-wallet watchonly model. Instead an entire wallet is considered to be
+watchonly depending on whether it was created with private keys disabled. This eliminates the need
+to distinguish between things that are watchonly and things that are not within a wallet itself.
+
+This change does have a caveat. If a Descriptor Wallet with private keys *enabled* has
+a multiple key descriptor without all of the private keys (e.g. `multi(...)` with only one private key),
+then the wallet will fail to sign and broadcast transactions. Such wallets would need to use the PSBT
+workflow but the typical GUI Send, `sendtoaddress`, etc. workflows would still be available, just
+non-functional.
+
+This issue is worsened if the wallet contains both single key (e.g. `wpkh(...)`) descriptors and such
+multiple key descriptors as some transactions could be signed and broadast and others not. This is
+due to some transactions containing only single key inputs, while others would contain both single
+key and multiple key inputs, depending on which are available and how the coin selection algorithm
+selects inputs. However this is not considered to be a supported use case; multisigs
+should be in their own wallets which do not already have descriptors. Although users cannot export
+descriptors with private keys for now as explained earlier.
+
+#### BIP 44/49/84 Support
+
+The change to using descriptors changes the default derivation paths used by Bitcoin Core
+to adhere to BIP 44/49/84. Descriptors with different derivation paths can be imported without
+issue.