internals: document that "branches" is a legacy wire command

Modern clients use a different discovery mechanism.

mercurial/help/internals/wireprotocol.txt

@@ -632,6 +632,9 b' There is no trailing newline.'
 branches
 --------
 
+(Legacy command used for discovery in old clients. Clients with ``getbundle``
+use the ``known`` and ``heads`` commands instead.)
+
 Obtain ancestor changesets of specific nodes back to a branch point.
 
 Despite the name, this command has nothing to do with Mercurial named branches.