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

glib/gmessages.c

@@ -180,6 +180,10 @@
  *
  * A convenience function/macro to log a warning message.
  *
+ * This is not intended for end user error reporting. Use of #GError is
+ * preferred for that instead, as it allows calling functions to perform actions
+ * conditional on the type of error.
+ *
  * You can make warnings fatal at runtime by setting the `G_DEBUG`
  * environment variable (see
  * [Running GLib Applications](glib-running.html)).
@@ -217,6 +221,10 @@
  *
  * A convenience function/macro to log an error message.
  *
+ * This is not intended for end user error reporting. Use of #GError is
+ * preferred for that instead, as it allows calling functions to perform actions
+ * conditional on the type of error.
+ *
  * Error messages are always fatal, resulting in a call to
  * abort() to terminate the application. This function will
  * result in a core dump; don't use it for errors you expect.