Add scenario where codeowners approval becomes optional

What does this MR do and why?

Updates docs to note that when merge request approval settings cause all codeowners to be ineligble to approve an MR, codeowner approval becomes optional instead of required.

For example, if the "Prevent approvals by users who add commits" setting is enabled on a project with one code owner and code owner approval required for changes to all branches, and that code owner adds a commit to any given MR, code owner approval for that MR will become optional instead of required.

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