Use a documented constant for the docutils levels

Instead of having a implicitly defined constant that doesn't
explain why the value was set use a documented frozenset that
has the valid docutils messaging levels with links/comments to
describe why they are being used.

Change-Id: I372c1babfb02c33b5881e2e6a12d464f75d944f7
Merge "Use a documented constant for the docutils levels"