Replace unnecessary with details

Author: zsxwing

language-adaptors/rxjava-scala/src/test/scala/rx/lang/scala/CompletenessTest.scala

@@ -77,8 +77,8 @@ class CompletenessTest extends JUnitSuite {
       "buffer(Observable[B])" -> "buffer(=> Observable[Any])",
       "buffer(Observable[B], Int)" -> "buffer(Observable[Any], Int)",
       "buffer(Observable[_ <: TOpening], Func1[_ >: TOpening, _ <: Observable[_ <: TClosing]])" -> "buffer(Observable[Opening], Opening => Observable[Any])",
-      "cast(Class[R])" -> unnecessary,
-      "collect(R, Action2[R, _ >: T])" -> unnecessary,
+      "cast(Class[R])" -> "[RxJava needs this one because `rx.Observable` is invariant. `Observable` in RxScala is covariant and does not need this operator.]",
+      "collect(R, Action2[R, _ >: T])" -> "foldLeft(R)((R, T) => R)",
       "contains(Any)" -> "contains(U)",
       "count()" -> "length",
       "debounce(Func1[_ >: T, _ <: Observable[U]])" -> "debounce(T => Observable[Any])",