Improve parsing of docblock specials

Summary:
Currently the tags of following docblock are parsed as array('special' => "foo\nbar"). This diff changes the output to be array('special' => array('foo', 'bar')).

/**
 * @special foo
 * @special bar
 */

Test Plan: Ran unit tests.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16431