test: add error equality tests + more warnings in docs

[hi-ogawa]

Description

• related toEqual/toStrictEqual does not check custom Error properties #5244

It doesn't look like there was a test case for error equality, so I added it.
I also added custom equality example to show how to achieve stricter behavior (like node:assert).

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
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Tests

• Run the tests with pnpm test:ci.

Documentation

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Changesets

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