Add info about differing UI and API behavior for access levels

What does this MR do?

Clarifies how access permissions work differently between the UI and the API when protecting branches.

The main addition explains that when using the API, you can set multiple permission levels at the same time (like "No access" and "Maintainer"), and the system will use the most permissive one. However, the UI automatically clears other selections when you choose "No one" for access.

This helps users understand why the API and web interface behave differently when managing who can modify protected branches in their code repositories.

Related issues

• Protected Branch API Patch request does not upd... (#551595)

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