doc: Unpublish wl_display_get_additional_shm_formats

The Wayland docbook and the doxygen html docs had been presenting
wl_display_get_additional_shm_formats as part of the public API, but the
prototype for this function is in wayland-private.h.

Add a \private annotation to the doc comment, preventing doxygen from
publishing this function as public.
Add logic to the publican xsl to only transform elements with a "prot"
attribute value of "public".

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