doc: Formalize file comment in wayland-client.h, wayland-server.h

Publican was generating a subtle error during a build:
Error: no ID for constraint linkend: Server-wayland-server-core_8h.

This was caused by doxygen applying the doc comment at the top of
wayland-server.h as the documentation for struct wl_object. As such, the
generated documentation for wl_object was also very incorrect.

Make the file doc comments in wayland-client.h and wayland-server.h real
doxygen file doc comments with the \file command, add a \brief, make the
inclusion warning a \warning, correct the language of the comment in
wayland-server.h, and remove one unnecessary line break.

This squelches the publican error, removes the bad wl_object documentation,
and makes the comment appear in the generated html documentation.

References: d74a9c07

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>