gerror: Document advantages of GError over numeric error codes

Despite knowing about GError, there are multiple cases where developers
have still used traditional numeric error codes, and then got themselves
into a mess about bindability and generation of error messages.

Try and avoid this by including a brief paragraph on the benefits of
GError over EINVAL-style error codes.

https://bugzilla.gnome.org/show_bug.cgi?id=743011