Merge bitcoin/bitcoin#21710: doc: update helps for addnode rpc and -addnode/-maxconnections config options

b4fcbcfb49461b96bc72fb64d6152de7c5ce00de doc: update -maxconnections config option help (Jon Atack)
79685a8992ad302833b506cc6d03aab1cc127de0 doc: update -addnode config option help (Jon Atack)
2896c6c4cc6d382d8369c037e274c08dd8e32c69 doc: update addnode rpc help (Jon Atack)

Pull request description:

  Since #9319 proposed by Gregory Maxwell and released in v0.14, peers manually added through the `-addnode` config option or using the `addnode` RPC have their own separate limit of 8 connections that does not compete with other inbound or outbound connection usage and is not subject to the limitation imposed by the `-maxconnections` option.

  This PR updates the `-addnode` and `-maxconnections` config options and the `addnode` RPC help docs with this information.

  `-addnode` config option help
  ```
  $ bitcoind -h | grep -A5 addnode=
    -addnode=<ip>
         Add a node to connect to and attempt to keep the connection open (see
         the addnode RPC help for more info). This option can be specified
         multiple times to add multiple nodes; connections are limited to
         8 at a time and are counted separately from the -maxconnections
         limit.

  $ bitcoind -h | grep -A3 maxconnections=
    -maxconnections=<n>
         Maintain at most <n> connections to peers (default: 125). This limit
         does not apply to connections manually added via -addnode or the
         addnode RPC, which have a separate limit of 8.
  ```

  `addnode` rpc help
  ```
  $ bitcoin-cli help addnode
  addnode "node" "command"

  Attempts to add or remove a node from the addnode list.
  Or try a connection to a node once.
  Nodes added using addnode (or -connect) are protected from DoS disconnection and are not required to be
  full nodes/support SegWit as other outbound peers are (though such peers will not be synced from).
  Addnode connections are limited to 8 at a time and are counted separately from the -maxconnections limit.
  ```

ACKs for top commit:
  prayank23:
    ACK https://github.com/bitcoin/bitcoin/commit/b4fcbcfb49461b96bc72fb64d6152de7c5ce00de
  jarolrod:
    ACK b4fcbcfb49461b96bc72fb64d6152de7c5ce00de

Tree-SHA512: b6d69baa6cbf6d53f91bac5b39b549d49db6c95f92ea1bdd3588a6432794a25ac2c8b3c89e2c72bb9097e61f2717c8b5ecc404745d5992b88e523db03200898f

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