document the need to be owner or have the master permission level for the initial push

What does this MR do?

It documents the new behavior as discussed on gitlab-org/gitlab-ce#22543, because the /protected_branch URL of empty repository is not available and could mislead users.

Are there points in the code the reviewer needs to double check?

Don't think so

Why was this MR needed?

When you create a repository on GitLab, command line instructions on its home page for the use case "Existing folder or Git repository" invite you to git push -u origin master, but the very first push is now rejected if you are not Owner or attributed Master permission.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

• new behavior was introduced here : gitlab-org/gitlab-ce@71dec8b1b61e9e194d242d37b39416b72020936b
• lack of documentation was discussed at gitlab-org/gitlab-ce#22543