Merge #19721: p2p: comment out unused MSG_FILTERED_WITNESS_BLOCK

4792cad doc: comment out and add annotation to unused MSG_FILTERED_WITNESS_BLOCK (Adam Jonas)

Pull request description:

  Commenting out and adding a note to unused `MSG_FILTERED_WITNESS_BLOCK` [defined in BIP144](https://github.com/bitcoin/bips/blob/master/bip-0144.mediawiki#relay).

  There was an attempt to make use of this in bitcoin/bitcoin#10350, but it was closed due to lack of support. (h/t sdaftuar for pointing to the PR and jnewbery for the idea)

ACKs for top commit:
  jnewbery:
    Obvious ACK 4792cad
  theStack:
    ACK bitcoin/bitcoin@4792cad 📜
  MarcoFalke:
    cr ACK 4792cad good to keep it around in a comment to avoid accidental future re-assignment
  practicalswift:
    ACK 4792cad

Tree-SHA512: 22327ddded643ae50fdb529e4529a9b464f74e90620d0d2079a11070eaa8afe8363f6e14cca52f3bec2c9f87ee13e318edc6c5193761c94b8ae77be353a8da1f

Author: fanquake

src/protocol.h

@@ -398,7 +398,9 @@ enum GetDataMsg : uint32_t {
     MSG_CMPCT_BLOCK = 4,                              //!< Defined in BIP152
     MSG_WITNESS_BLOCK = MSG_BLOCK | MSG_WITNESS_FLAG, //!< Defined in BIP144
     MSG_WITNESS_TX = MSG_TX | MSG_WITNESS_FLAG,       //!< Defined in BIP144
-    MSG_FILTERED_WITNESS_BLOCK = MSG_FILTERED_BLOCK | MSG_WITNESS_FLAG,
+    // MSG_FILTERED_WITNESS_BLOCK is defined in BIP144 as reserved for future
+    // use and remains unused.
+    // MSG_FILTERED_WITNESS_BLOCK = MSG_FILTERED_BLOCK | MSG_WITNESS_FLAG,
 };
 
 /** inv message data */