docs: say the Action blocks a merge only once it is required

The Action guide opened with "A CI check cannot [be skipped]". A check that
is not required can be: the pull request merges with it red. The page never
said how to make it required, though the landing page and the App guide do.

The opening now says what a local hook and the Action each do, and a short
section says where to require the check and what it is called. GitHub names
an Action's check after its job, so with the workflow on the page it is
commit-check.

Author: shenxianpeng

docs/guides/github-actions.md

@@ -1,7 +1,9 @@
 # GitHub Action
 
-Local hooks can be skipped. A CI check cannot, so the Action is where the
-policy is enforced.
+A local hook can be skipped with `git commit --no-verify`. The Action runs in
+CI whatever happened locally, and once it is a
+[required status check](#making-it-a-required-check) a pull request that
+fails it cannot merge.
 
 ```yaml title=".github/workflows/commit-check.yml"
 name: Commit Check
@@ -35,6 +37,13 @@ jobs:
 The Action reads the same `cchk.toml` as the CLI, so a repository that already
 has one needs no Action-specific configuration.
 
+## Making it a required check { #making-it-a-required-check }
+
+Until it is required, a failing Commit Check does not stop anyone from
+merging. Require it in the branch's ruleset or branch protection, where the
+repository's plan allows one. GitHub names an Action's check after its job, so
+with the workflow above the check to require is `commit-check`.
+
 ## Commenting on the pull request
 
 Instead of making contributors open the job log, have the Action post what