Provider cross version checks warning

Add a warning note to the provider cross version checks indicating that a
pull request branch will not be used if execution is set to on pull request.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21964)

.github/workflows/provider-compatibility.yml

@@ -10,6 +10,10 @@
 
 name: Provider compatibility across versions
 
+
+# NOTE: if this is being run on pull_request, it will **not** use the pull
+#       request's branch.  It is hardcoded to use the master branch.
+#
 on: #[pull_request]
   schedule:
     - cron: '0 15 * * *'