wireproto: document quirk of _callstream between http and ssh

This tripped me up when trying to use it, so it feels like we should
document this to avoid future pain.

mercurial/wireproto.py

@@ -396,9 +396,12 b' class wirepeer(peer.peerrepository):'
     def _callstream(self, cmd, **args):
         """execute <cmd> on the server
 
-        The command is expected to return a stream.
+        The command is expected to return a stream. Note that if the
+        command doesn't return a stream, _callstream behaves
+        differently for ssh and http peers.
 
-        returns the server reply as a file like object."""
+        returns the server reply as a file like object.
+        """
         raise NotImplementedError()
 
     def _callcompressable(self, cmd, **args):