Document predefined branch rules

What does this MR do and why?

Adding documentation for handling "All branches" and "All protected branches" brach rules.

Feature issue: #388129 MR with implementation: !148412 (merged)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

n/a

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

n/a

Related to #388129