Document Mercurial base commit DSL support

Summary: documentation changes for D2822

Test Plan:
Generated docs, but failed to read them because no one at Facebook
remembers how to get docs working in our sandbox.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1233

Differential Revision: https://secure.phabricator.com/D2827