Fix `rules:exists` example and description

What does this MR do?

The current explanation for the example YAML in the rules:exists: section is incorrect – no glob is used, so we're not looking anywhere in the repo, just at the root directory. I noticed this when looking through !149753 which copied that error.

Added an example that does what the current description says, probably doesn't hurt to illustrate globs some more.

Related issues

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