doc/common.txt

check-code: automatically preppend "warning: " to all warning messages

Some warnings had "warning: " at the beginning of their message. Now this
is done consistent for all messages.
Especially in test-check-code-hg.t it is an advantage to see warnings at once
because only exceptions to them are tolerated. It is (almost) as obvious as
before a6180647ea.

The prefix will not remain when a warning is changed to a failure. A change
like a91387a37f will not be necessary anymore.

.. 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