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

src/docs/userguide/arcanist_commit_ranges.diviner

@@ -82,8 +82,13 @@ is found. Valid rules are listed below. In this list, "*" means "any string".
   - `git:*` Use the specified symbolic commit, if it exists.
   - `git:merge-base(*)` Use the merge-base of HEAD and the specified symbolic
     commit, if it exists.
+  - `hg:*` Use the specified symbolc commit, if it exists.
+  - `hg:gca(*)` Use the greatest common ancestor of `.` and the specified
+     symbolic commit, if it exists.
   - `arc:upstream` Use the merge-base of the current branch's upstream and
-    HEAD, if it exists.
+     HEAD, if it exists. (git-only)
+  - `arc:outgoing` Use the most recent non-outgoing ancestor of the working
+     copy parent. (hg-only)
   - `arc:prompt` Prompt the user to provide a commit.
   - `arc:empty` Use the empty state (as though the repository were completely
     empty, the range will include every commit that is an ancestor of the