gmessages: Mention g_return_if_fail() in g_warning() and g_error() docs

It seems to be common for people to use g_warning() or g_error() as pre-
and post-condition error reporting functions, which is not really what
they’re intended for. Similarly, it is generally a sign of bad API
design to use g_warning() to report errors — use GError instead.

Try and suggest this to the user in the hope that nice code results.

https://bugzilla.gnome.org/show_bug.cgi?id=741779