Makefile: add nit-picky mode to sphinx-build

If we add references that don't resolve (or accidentally remove them),
it will be helpful to have warning messages alerting us to that.

Further, turn those warnings into errors so we can be alerted to these
problems sooner rather than later.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20190426221528.30293-2-jsnow@redhat.com
[adjusted commit message. --js]
Signed-off-by: John Snow <jsnow@redhat.com>