doc: Add some doxygen documentation to wayland-client entry points

Add some brief documentation for the public libwayland-client entry
points. This is by no means complete, some functions are still
undocumented and some might need extra information.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Tiago Vignatti <tiago.vignatti@intel.com>