Improve Docblock parser handling of "@flags"

Summary:
Allow the docblock parser to handle "@specials" with no value, like "@stable",
more gracefully, by treating them as flags.

Also improve extraction of docblock text from docblocks with mixed-order
specials and text.

Test Plan: This class is extensively covered by unit tests.

Reviewers: btrahan, nh, jungejason

Reviewed By: nh

CC: aran, epriestley

Maniphest Tasks: T795

Differential Revision: https://secure.phabricator.com/D1508