xdg-shell: Specify fullscreen size-mismatch handling

We cannot rely on the client to provide a surface filling the output so
we must specify what happens with the outputs area that is not covered
completely.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

protocol/xdg-shell.xml

@@ -403,6 +403,10 @@
         You can specify an output that you would prefer to be fullscreen.
 	If this value is NULL, it's up to the compositor to choose which
         display will be used to map this surface.
+
+        If the surface doesn't cover the whole output, the compositor will
+        position the surface in the center of the output and compensate with
+        black borders filling the rest of the output.
       </description>
       <arg name="output" type="object" interface="wl_output" allow-null="true"/>
     </request>