Document how to handle tolerances

[OsirisTerje]

See issue in the framework as follows:

• CollectionTally (EquivalentTo) should throw for non-transitive comparisons nunit#2870
• Ensure tolerance is available to EqualityAdapters and is not silently ignored nunit#4312
• Provide Asserts for System.Numerics types nunit#3316
• .Within() can be silently ignored, causing surprises nunit#4310

[mr-russ]

What sort of documentation are you considering? I'm not sure I understand the scope of the problem are solving just yet.

[OsirisTerje]

When we figure out all, and have implemented all, we need to ensure that https://docs.nunit.org are up to date with the changes. The Within is sparsely documented as is.
PS: That goes for many of the other modifiers as well.