Document some AssertJ equivalents, especially for Iterable assertions

[cpovirk]

AssertJ's AbstractIterableAssert is large. We don't have an equivalent to every last assertion (and this is sometimes intentional), but for cases in which we do have an equivalent, we could help prospective users / migrating users / users with an AssertJ background by providing a mapping between the two.

A lot of advanced assertions would use Fuzzy Truth. For example, AssertJ assertions that use areAtLeastOne(condition) and extractingResultOf(method) would often use comparingElementsUsing.