util: Document wl_fixed_t

Add doxygen comments for wl_fixed_t and its methods.

Although wl_fixed_t can be thought of as an opaque struct, it is a typedef. As
such, doxygen does not provide an elegant means of documenting it as both a
'class' with members and as a typedef. In other words, documenting it as a class
gives us a nice doxygen page for wl_fixed_t and its related methods, but this
leaves the typedef documentation blank in the documentation for wayland-util,
and does not provide a link to the documentation for wl_fixed_t. Hence, this
patch does not treat wl_fixed_t as a class/struct, resulting in the typedef
being documented and keeping the functions listed in wayland-util, rather than a
separate unlinked page devoted to just wl_fixed_t.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Daniel Stone <daniels@collabora.com>