add some more Scaladoc

Author: SethTisue

src/main/scala/scala/collection/immutable/next/package.scala

@@ -24,6 +24,10 @@ package object next {
      *
      * Safe to call on unbounded input, but in that case the result is not a cycle
      * (not even if the input was).
+     *
+     * Note that some `LazyList` methods preserve cyclicality and others do not.
+     * So for example the `tail` of a cycle is still a cycle, but `map` and `filter`
+     * on a cycle do not return cycles.
      */
     def cycle: LazyList[T] =
       // case 1: the input is already known to be empty