Refine handling of "@task" attributes in Diviner

Summary: Ref T13505. See that task for details. When a class has exactly one "@task" block, this API returns a string. Some day, this should be made more consistent.

Test Plan: Viewed a class with exactly one "@task", no more fatal. Viewed classes with zero and more than one "@task" attributes, got clean renderings.

Maniphest Tasks: T13505

Differential Revision: https://secure.phabricator.com/D21062