Improving protect repository branches API documentation

What does this MR do?

It fixes the type for push_access_level, merge_access_level, and unprotect_access_level, it should be integer.

It adds inline what is the expected form of a hash for allowed_to_push, allowed_to_merge, and allowed_to_unprotect. It is mentioned later on, but I find it more useful if we show it immediately. It took me some time to understand it without this.

Review checklist

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