wireprotoserver: rename _client to client (API)
This method is called in wireproto.py. It should be part of the public
API/interface.
.. api::
The ``_client()`` method of the wire protocol handler interface has
been renamed to ``client()``.
Differential Revision: https://phab.mercurial-scm.org/D2084
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors