Allow the docblock parser to handle hyphens in "@specials"

Summary: Trying to add one named "@phutil-external-symbol" and it doesn't work
right now because "-" is not in \w.

Test Plan: Unit tests.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, jungejason

Maniphest Tasks: T762

Differential Revision: https://secure.phabricator.com/D1380