Improve documentation of RegExp in CI/CD job ref patterns

What does this MR do?

• correct one example of RegExp use for matching ref names in CI/CD job definitions
• add some more notes and tips for how RegExp behave in that context

Why?

The current documentation was misleading w.r.t. that topic, especially when the patterns also include a repo path. See

• #30477 (moved), particularly https://gitlab.com/gitlab-org/gitlab-ce/issues/30477#note_29060568
• .gitlab-ci.yml: How to specify “only/except” rules with a regex AND repo-specific qualifier? on StackOverflow

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content (none)

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team