wireproto: document possible return type

The wireprotocole command use a small set of class as return value. We document
the meaning of each of them.

AS my knowledge of wire protocol is fairly shallow, the documentation can
probably use improvement. But this is a better than nothing.

mercurial/wireproto.py

@@ -327,19 +327,38 b' class wirepeer(peer.peerrepository):'
 
 # server side
 
+# wire protocol command can either return a string or one of these classes.
 class streamres(object):
+    """wireproto reply: binary stream
+
+    The call was successful and the result is a stream.
+    Iterate on the `self.gen` attribute to retrieve chunks.
+    """
     def __init__(self, gen):
         self.gen = gen
 
 class pushres(object):
+    """wireproto reply: success with simple integer return
+
+    The call was successful and returned an integer contained in `self.res`.
+    """
     def __init__(self, res):
         self.res = res
 
 class pusherr(object):
+    """wireproto reply: failure
+
+    The call failed. The `self.res` attribute contains the error message.
+    """
     def __init__(self, res):
         self.res = res
 
 class ooberror(object):
+    """wireproto reply: failure of a batch of operation
+
+    Something failed during a batch call. The error message is stored in
+    `self.message`.
+    """
     def __init__(self, message):
         self.message = message