diagnostic: reword issue reporting guidelines

[scpeters]

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I recently saw a brew doctor complaint about an outdated command line tools with the following support tier message:

This is a Tier 2 configuration:
  https://docs.brew.sh/Support-Tiers#tier-2
You can report Tier 2 unrelated issues to Homebrew/* repositories!
Read the above document before opening any issues or PRs.

I find the phrasing of "You can report Tier 2 unrelated issues" a bit confusing (in particular the word unrelated), and my best guess for alternate phrasing is "You can report issues with Tier 2 configurations" so I opened a PR to suggest that wording.

If you prefer the current wording, can you clarify if it means:

• You can report issues observed on a system with a Tier 2 configuration that are unrelated to the diagnostic complaint that triggered this message
• You can report issues unrelated to your Tier 2 configuration
• Something else

Thanks!

[MikeMcQuaid]

Makes sense, thanks!