Add assertion error docs

This was never properly documented but it is used in the wild and it makes sense to actually explain how it works and what properties the errors contain. Those properties can be useful for the users.

Please note: I explicitly did not document the errorDiff option as I plan on removing it again in 10.x. It was introduced in v.9.9.x and I think we can still go ahead to remove it as it is unlikely that it is already used.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines