man: document WESTON_LIBINPUT_LOG_PRIORITY env

Commit c81c4241 added this environment
variable. Document it.

It applies also to the fbdev-backend but that has no man page.

v2:
- Rewording by Peter Hutterer.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>