Add flatMap sample KT-20357

libraries/stdlib/common/src/generated/_Strings.kt

@@ -749,6 +749,8 @@ public fun CharSequence.toSet(): Set<Char> {
 
 /**
  * Returns a single list of all elements yielded from results of [transform] function being invoked on each character of original char sequence.
+ * 
+ * @sample samples.collections.Collections.Transformations.flatMap
  */
 public inline fun <R> CharSequence.flatMap(transform: (Char) -> Iterable<R>): List<R> {
     return flatMapTo(ArrayList<R>(), transform)