Mention tag pipeline limitation for protected tags

What does this MR do?

When working on 🎫 #565334 (internal) I noticed that we don't mention how protected tags impact pipeline execution permissions in the same way that protected branches do. For branches we have this section: https://docs.gitlab.com/ee/user/project/protected_branches.html#run-pipelines-on-protected-branches

Related issues

Our tag pipeline docs were lacking a bit in general, see Add tag pipeline documentation (!165508 - merged) as well.

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