Minor doc updates

Summary:
Sort all the symbols in libphutil into groups.

Also fix a broken unit test -- under normal PHP, we can't assign $log, or it won't be destructed until the end of the function.

Test Plan: Generated, read documentation.

Reviewers: btrahan, edward, jungejason

Reviewed By: jungejason

CC: aran

Differential Revision: https://secure.phabricator.com/D2462