Add an explanation of the autoupdate-branch workflow (#17798)

Author: JamesMGreene

.github/workflows/autoupdate-branch.yml

@@ -1,8 +1,23 @@
 name: autoupdate branch
+
+#
+# This workflow checks all open PRs targeting `main` as their base branch and
+# will attempt to update them if they have the `autoupdate` label applied.
+# It is triggered when a `push` event occurs ON the `main` branch (e.g. a PR
+# was merged or a force-push was done).
+#
+# It should work on all PRs created from source branches within the repo itself
+# but is unlikely to work for PRs created from forked repos.
+#
+# It is still worthwhile to leave it enabled for the `docs` open source repo as
+# it should at least be running on `repo-sync` branch PRs.
+#
+
 on:
   push:
     branches:
       - main
+
 jobs:
   autoupdate:
     if: github.repository == 'github/docs-internal' || github.repository == 'github/docs'