Use new format for docblock specials when parsing symbols

Summary: Fixes T11575. Before, specials used to be separated by newlines, now they are returned as an array for us, or returned as a single value for us. Casting the result to an array is the minimum amount of disruption we can incur here to keep the existing logic in place and provide the same values.

Test Plan:

• deleted my src/.phutil_module_cache from arcanist, libphutil and phabricator
• ran arc liberate src in all of those places
• ran all of their unit tests
• saw all tests pass and no modifications made to __phutil_library_map__.php for any of the libraries

Reviewers: #blessed_reviewers, epriestley, joshuaspence

Reviewed By: #blessed_reviewers, epriestley, joshuaspence

Subscribers: epriestley

Maniphest Tasks: T11575

Differential Revision: https://secure.phabricator.com/D16484