Make PhutilDocblockParser slightly more liberal in finding "@special" annotations

Summary:
Ref T988. XHProf has some docblocks like this:

  /**
   * Blah blah blah, one space indent.
   *
   *  @author two space indent
   */

This is slightly ambiguous with code blocks, but be more liberal in parsing it since we don't really create any ambiguity.

Test Plan: Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6811