Formalize a mechanism for marking Conduit methods deprecated/unstable

Summary:
This is better than writing "(UNSTABLE!!!)" in front of the text description.

I'll add a wiki to keep track of API changes, too.

See also D2087, which motivates this.

Test Plan: Browsed console, saw "deprecated" and "unstable" on appropriate methods.

Reviewers: btrahan, vrana, jungejason

Reviewed By: vrana

CC: aran

Maniphest Tasks: T909

Differential Revision: https://secure.phabricator.com/D2271