changegroup: reformat packermap and add comment

I'm about to add a cg3, and it seems prudent to annotate what formats
support what features. It strikes me that we may want to consider
moving to a more feature-oriented model in the future, but we'll see
how that looks in a little while I guess.

mercurial/changegroup.py

@@ -776,7 +776,9 b' class cg2packer(cg1packer):'
         return struct.pack(self.deltaheader, node, p1n, p2n, basenode, linknode)
 
 packermap = {'01': (cg1packer, cg1unpacker),
-             '02': (cg2packer, cg2unpacker)}
+             # cg2 adds support for exchanging generaldelta
+             '02': (cg2packer, cg2unpacker),
+}
 
 def _changegroupinfo(repo, nodes, source):
     if repo.ui.verbose or source == 'bundle':