The wording of Single.flattenAsFlowable is a bit off:

"Returns a Flowable that merges each item emitted by the source Single with the values in an Iterable corresponding to that item that is generated by a selector."

In contrast, flattenAsObservable is somewhat better:

"Returns an Observable that maps a success value into an Iterable and emits its items."

I'd say something along the lines of:

"Maps the success value of the upstream {@code Single} into an {@link Iterable} and emits its items as an {@link Observable} sequence."

The Maybe variants have the same issues (they were copy-pasted back then).