Document MonoFlatMapToFlux Refaster rule limitation (PicnicSupermar…

…ket#473)

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/ReactorRules.java

@@ -474,6 +474,11 @@ Flux<S> after(
    * Flux}.
    */
   abstract static class MonoFlatMapToFlux<T, S> {
+    // XXX: It would be more expressive if this `@Placeholder` were replaced with a `Function<?
+    // super T, ? extends Mono<? extends S>>` parameter, so that compatible non-lambda expression
+    // arguments to `flatMapMany` are also matched. However, the type inferred for lambda and method
+    // reference expressions passed to `flatMapMany` appears to always be `Function<T, Publisher<?
+    // extends S>>`, which doesn't match. Find a solution.
     @Placeholder(allowsIdentity = true)
     abstract Mono<S> transformation(@MayOptionallyUse T value);