NUnit 5 | Document that ThrowsAsync returns a Task

[stevenaw]

Version 5 of NUnit changes the signature of the following:

• ThrowsAsync
• CatchAsync
• DoesNotThrowAsync

To return an awaitable. Documentation should be updated to reflect the new signature. The PR which introduces the change: nunit/nunit#5268

Example pages:

• https://docs.nunit.org/articles/nunit/writing-tests/assertions/classic-assertions/Assert.ThrowsAsync.html
• https://docs.nunit.org/articles/nunit/writing-tests/assertions/classic-assertions/Assert.CatchAsync.html
• https://docs.nunit.org/articles/nunit/writing-tests/assertions/classic-assertions/Assert.DoesNotThrowAsync.html