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>