Specify regexp language used for `only`/`except` mechanism in `.gitlab-ci.yaml`

doc/ci/yaml/README.md

@@ -334,7 +334,7 @@ There are a few rules that apply to the usage of job policy:
 
 * `only` and `except` are inclusive. If both `only` and `except` are defined
    in a job specification, the ref is filtered by `only` and `except`.
-* `only` and `except` allow the use of regular expressions.
+* `only` and `except` allow the use of regular expressions (using [Ruby regexp syntax](https://ruby-doc.org/core/Regexp.html)).
 * `only` and `except` allow to specify a repository path to filter jobs for
    forks.