changegroup: document that 'source' parameter exists for extensions

The 'source' parameter passed to generatefiles() is unused by the
method itself, but Durham says it is used by an extension.

mercurial/changegroup.py

@@ -440,6 +440,7 b' class cg1packer(object):'
         if clnodes:
             repo.hook('outgoing', node=hex(clnodes[0]), source=source)
 
+    # The 'source' parameter is useful for extensions
     def generatefiles(self, changedfiles, linknodes, commonrevs, source):
         repo = self._repo
         progress = self._progress