ci: correct the companion-workflow comments

The ci-docs-only header claimed the ruleset already requires "Quick Checks";
it does not, and this PR deliberately leaves it that way. Say what is true
today and why the job is mirrored ahead of the ruleset change.

Also move the byte-identical requirement onto ci.yml's quick-checks job, which
is the more likely edit site, instead of pointing at a comment that was not
there.

.github/workflows/ci.yml

@@ -96,6 +96,10 @@ jobs:
 
   # Fast, compile-free checks that fail early so contributors get feedback in
   # ~1 minute instead of waiting for the full test job.
+  #
+  # These steps are mirrored byte-for-byte in ci-docs-only.yml so that a mixed
+  # PR, which reports two check runs named "Quick Checks", cannot get one red
+  # and one green. Edit both jobs together.
   quick-checks:
     name: Quick Checks
     if: github.event_name != 'pull_request' || github.event.action != 'closed'