Improve hint for repository access

What does this MR do and why?

In discussion (internal Slack) around how to make a repository private whilst a project was public, it was identified that the hint copy for repository settings (http://gdk/group-name/project-name/edit) was a little unclear.

It's unclear because the hint does not change even if the dropdown value does, e.g. to Only Project Members

This MR attempts to address it by clarifying that the read-only access is a default.

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