tox: Silence docs build

* Add "--quiet" to the docs build: otherwise it drowns out everything
  else when running "tox"
* switch other short arguments to long ones as well for clarity

Signed-off-by: Jussi Kukkonen <jkukkonen@google.com>

Author: jku

tox.ini

@@ -54,4 +54,4 @@ deps =
     -r{toxinidir}/requirements/docs.txt
 
 commands =
-    sphinx-build -b html docs docs/build/html -W
+    sphinx-build --fail-on-warning --quiet --builder html docs docs/build/html