Document some AssertJ equivalents, especially for Iterable assertions

AssertJ's [`AbstractIterableAssert`](https://joel-costigliola.github.io/assertj/core/api/org/assertj/core/api/AbstractIterableAssert.html) is large. We don't have an equivalent to every last assertion (and this is [sometimes intentional](https://truth.dev/comparison#assertion-count)), 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](https://truth.dev/fuzzy). For example, AssertJ assertions that use `areAtLeastOne(condition)` and `extractingResultOf(method)` would often use `comparingElementsUsing`.