bundle2: add an informative comment to the capability dict

It is fairly easy to get confused by capabilities "missing" from this dict. We
make it clear the dict is not the whole story.

mercurial/bundle2.py

@@ -1019,6 +1019,8 b' class unbundlepart(unpackermixin):'
                 raise util.Abort(_('Seek failed\n'))
             self._pos = newpos
 
+# These are only the static capabilities.
+# Check the 'getrepocaps' function for the rest.
 capabilities = {'HG20': (),
                 'listkeys': (),
                 'pushkey': (),