Makefile: add nit-picky mode to sphinx-build

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>
(cherry picked from commit 9e5b6cb87db66dfb606604fe6cf40e5ddf1ef0e7)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>