Provide optional detailed help from workflows

Summary: Acts like "synopsis" but is shown only on detailed help. If you don't want synopsii (sp?), you could just call setHelp() instead of setSynopsis(). Seem reasonable?

Test Plan: Faked some help, ran `x help`, `x help y`.

Reviewers: davidreuss

Reviewed By: davidreuss

CC: aran

Differential Revision: https://secure.phabricator.com/D4950