doc/common.txt

doc: fix careless document miss in help of hgext/notify

according to configuration example below, and direction of changeset
transference, this paragraph should describe about "changegroup" hook.

  [hooks]
  # one email for each incoming changeset
  incoming.notify = python:hgext.notify.hook
  # one email for all incoming changesets
  changegroup.notify = python:hgext.notify.hook

  # one email for all outgoing changesets
  outgoing.notify = python:hgext.notify.hook

.. Common link and substitution definitions.

.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html