libweston: prevent double weston_output_enable()

Enabling an already enabled output is an error, at least with the
current implementation.

However, disabling an output that has not been enabled is ok.

Cope with the first and document the second.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Armin Krezović <krezovic.armin@gmail.com>