Document using --verbosity 3 which maps to -vv (#403)

Author: ekohl

docs/faq.rst

@@ -80,6 +80,8 @@ test runner like this::
                 argv.append('--quiet')
             if self.verbosity == 2:
                 argv.append('--verbose')
+            if self.verbosity == 3:
+                argv.append('-vv')
             if self.failfast:
                 argv.append('--exitfirst')
             if self.keepdb: