Document differences in coverage ignore comments between providers

[spenserblack]

Summary

It wasn't immediately clear how to ignore lines from the coverage report, and where to find the documentation on ignore comments.

This adds a note to the coverageProvider configuration option providing the basic ignore comments to ignore a line, and also permalinks to documentation for more complex ignore comment usage.

Addresses #12835

Test plan

Only documentation was changed, so likely N/A 🤔

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[spenserblack]

I found my changes in f14e29a to be overly wordy once I previewed, but I left the commit in there in case that level of detail is preferred after all.

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[mrazauskas]

Sounds good. Reads well next to coverageProvider, but it feels like this note would be more useful next to collectCoverage. Only that it would be strange to talk about babel / v8 there.

Just thinking out loud, feel free to rework. What if collectCoverage documentation would be extended like this:

  ### `collectCoverage` \[boolean]

  Default: `false`

  Indicates whether the coverage information should be collected while executing the test. Because this retrofits all executed files with coverage collection statements, it may significantly slow down your tests.

+ Jest ships with two coverage providers: `babel` (default) and `v8`. See [`coverageProvider`](#coverageProvider) option for more details.
+
+ :::note
+ 
+ The `babel` and `v8` providers use `/* istanbul ignore next */` and `/* c8 ignore next */` comments to exclude lines from coverage reports, respectively. For more information, you can view the [`istanbuljs` documentation](https://github.com/istanbuljs/nyc#parsing-hints-ignoring-lines) and the [`c8` documentation](https://github.com/bcoe/c8#ignoring-uncovered-lines-functions-and-blocks).
+ 
+ :::

[spenserblack]

What if collectCoverage documentation would be extended like this:

Yeah, it does look better there. Definitely easier to find, since that's probably where users will first look when setting up coverage. I'll move the comment and apply your suggestions.

[mrazauskas]

Thanks! Looks good for my eye.

[SimenB]

perfect, thanks!

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