Add note on coverage report limits to troubleshooting section

What does this MR do and why?

🎟️ Add note on coverage report limits to troubleshooting section

I am opening this documentation-only MR based on something that came up in a 🎫 a ticket. The customer observed that having this information in the troubleshooting section of the test coverage visualization docs would have helped them to more rapidly identify the cause of the problem they encountered.

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