Document Assert.EnterMultipleScope

[manfred-brands]

NUnit 4.2 adds IDisposable EnterMultipleScope() method in addition to the Assert.Multiple() delegate.

This issue is to update the documentation with this.

NUnit PR 4587

[chm-tm]

Document EnterMultipleScope as single way to accomplish multiple asserts, due to its advantages over Assert.Multiple.

In order to remove an unnecessary option and lower the learning curve, go so far as to mark Assert.Multiple as legacy and that it will be removed in a future release, also providing a code fixer to transform Assert.Multiple to Assert.EnterMultipleScope. This is in the same spirit as the braking change around classic asserts.

If the discussion about this suggestion leads toward permanent coexistence of both APIs, document EnterMultipleScope first.