xdg-output: clarify goal
In the past, xdg-output had only the logical_position and logical_size events, then the name and description events were added. Later on, they were moved inside wl_output since they aren't desktop-specific. However the goals section of the protocol overview hasn't been updated accordingly.
Make it clear that this protocol's name and description events should not be preferred over wl_output.
Signed-off-by: Simon Ser contact@emersion.fr