diff --git a/src/docs/userguide/arcanist_commit_ranges.diviner b/src/docs/userguide/arcanist_commit_ranges.diviner
index 9f634b6bad..17b5655225 100644
--- a/src/docs/userguide/arcanist_commit_ranges.diviner
+++ b/src/docs/userguide/arcanist_commit_ranges.diviner
@@ -82,13 +82,18 @@ 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:*` Use the specified symbolic 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. (git-only)
   - `arc:outgoing` Use the most recent non-outgoing ancestor of the working
      copy parent. (hg-only)
+  - `arc:exec(*)` Execute the specified command. The command should determine
+    the base revision to use and print it to stdout, then exit with return code
+    0. If the command exits with another return code, the rule will fail. The
+    command will be executed with the root directory of the working copy as the
+    current working directory.
   - `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