[PATCH 2 of 7 iterbatch] wireproto: document quirk of _callstream between http and ssh

[Augie Fackler]

# HG changeset patch
# User Augie Fackler <augie at google.com>
# Date 1456946323 18000
#      Wed Mar 02 14:18:43 2016 -0500
# Node ID 071d05cce9156434dee4696287119f284631701b
# Parent  5fd58c8c55d052c671a4725d01f27fb1c14353a9
# EXP-Topic batch
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.

diff --git a/mercurial/wireproto.py b/mercurial/wireproto.py
--- a/mercurial/wireproto.py
+++ b/mercurial/wireproto.py
@@ -396,9 +396,12 @@ 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):