Mark all libphutil stable classes as @stable
Summary: Provide an explicit hint about stability/extensibility for
documentation.
Test Plan: Regenerated documentation, looked at @stable and not-@stable classes.
Reviewers: btrahan, jungejason
Reviewed By: btrahan
CC: aran, epriestley
Maniphest Tasks: T795
Differential Revision: https://secure.phabricator.com/D1502