Merge #18787: wallet: descriptor wallet release notes and cleanups

ca2a09640fe976b1e74a33d29d9381895e71b347 Change SetType to SetInternal and remove m_address_type (Andrew Chow)
89b1ce1140535b4c902a7c5999bed335b9ddfe7c Remove unimplemented SetCrypted from DescriptorScriptPubKeyMan (Andrew Chow)
b9073c8f13fb0ba94c2ec6365666343e19fd9ddf rpc: createwallet warning that descriptor wallets are experimental (Andrew Chow)
610030d95c60ea526440d801a98ac8bd370eac48 docs: Add release notes for descriptor wallets (Andrew Chow)

Pull request description:

  Some docs and cleanup following #16528.

  * Added release notes to explain a bit of motivation for descriptor wallets, what was changed, and how users will be effected by it. Also mentions the caveats regarding multsigs and watchonly that we have discussed on IRC.
  * Adds a warning to `createwallet` that descriptor wallets are experimental.
  * Removed unused `SetCrypted` as suggestioned: https://github.com/bitcoin/bitcoin/pull/16528#discussion_r415300916
  * Removed `m_address_type` as mentioned in https://github.com/bitcoin/bitcoin/pull/18782#issuecomment-620167077

ACKs for top commit:
  Sjors:
    tACK ca2a09640fe976b1e74a33d29d9381895e71b347
  instagibbs:
    utACK https://github.com/bitcoin/bitcoin/commit/ca2a09640fe976b1e74a33d29d9381895e71b347
  meshcollider:
    utACK ca2a09640fe976b1e74a33d29d9381895e71b347

Tree-SHA512: 987188a912c191430e5d3f89bcef54ba6773692fc2d95b16a3ec11d9007ded210466ed980a3857e8b7196beef6422f07f9c85cc157f996c02d16f4dbde2e7b2a

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