[PATCH] internals: document that "branches" is a legacy command

[Siddharth Agarwal]

# HG changeset patch
# User Siddharth Agarwal <sid0 at fb.com>
# Date 1493845634 25200
#      Wed May 03 14:07:14 2017 -0700
# Node ID 7e90b9553a02ef30fd289736cfb559c83b6766e2
# Parent  8ac7ac714e922fd34abe9f5a16fcf018c8b12abc
internals: document that "branches" is a legacy command

Modern clients use a different discovery mechanism.

diff --git a/mercurial/help/internals/wireprotocol.txt b/mercurial/help/internals/wireprotocol.txt
--- a/mercurial/help/internals/wireprotocol.txt
+++ b/mercurial/help/internals/wireprotocol.txt
@@ -632,6 +632,9 @@ 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.