[PATCH 1 of 4 V2] bundle2: add documention to 'part.addparams'

[Pierre-Yves David]

# HG changeset patch
# User Pierre-Yves David <pierre-yves.david at ens-lyon.org>
# Date 1491754569 -7200
#      Sun Apr 09 18:16:09 2017 +0200
# Node ID 75799788617866ec78984b08225e025eeca73790
# Parent  e0dc40530c5aa514feb6a09cf79ab6a3aa2ec331
# EXP-Topic bundle2.doc
# Available At https://www.mercurial-scm.org/repo/users/marmoute/mercurial/
#              hg pull https://www.mercurial-scm.org/repo/users/marmoute/mercurial/ -r 757997886178
bundle2: add documention to 'part.addparams'

There are some non-obvious limitations on the parameters of this method.
Add some documentation where people will likely look to understand how
to use this API.

diff --git a/mercurial/bundle2.py b/mercurial/bundle2.py
--- a/mercurial/bundle2.py
+++ b/mercurial/bundle2.py
@@ -900,6 +900,13 @@ class bundlepart(object):
         return tuple(self._advisoryparams)
 
     def addparam(self, name, value='', mandatory=True):
+        """add a parameter to the part
+
+        If 'mandatory' is set to True, the remote handler must claim support
+        for this parameter or the unbundling will be aborted.
+
+        The 'name' and 'value' cannot exceed 255 bytes each.
+        """
         if self._generated is not None:
             raise error.ReadOnlyPartError('part is being generated')
         if name in self._seenparams: