KT-20357 Add sample for mutableSet function

libraries/stdlib/samples/test/samples/collections/collections.kt

@@ -311,6 +311,28 @@ class Collections {
             // but the sets with the same elements are equal no matter of order
             assertTrue(set1 == set2)
         }
+
+        @Sample
+        fun emptyMutableSet() {
+            val set = mutableSetOf<Int>()
+            assertTrue(set.isEmpty())
+
+            set.add(1)
+            set.add(2)
+            set.add(1)
+
+            assertPrints(set, "[1, 2]")
+        }
+
+        @Sample
+        fun mutableSet() {
+            val set = mutableSetOf(1, 2, 3)
+            assertPrints(set, "[1, 2, 3]")
+
+            set.remove(3)
+            set += listOf(4, 5)
+            assertPrints(set, "[1, 2, 4, 5]")
+        }
     }
 
     class Transformations {

libraries/stdlib/src/kotlin/collections/Sets.kt

@@ -52,6 +52,7 @@ public inline fun <T> setOf(): Set<T> = emptySet()
  * Returns an empty new [MutableSet].
  *
  * The returned set preserves the element iteration order.
+ * @sample samples.collections.Collections.Sets.emptyMutableSet
  */
 @SinceKotlin("1.1")
 @kotlin.internal.InlineOnly
@@ -60,6 +61,7 @@ public inline fun <T> mutableSetOf(): MutableSet<T> = LinkedHashSet()
 /**
  * Returns a new [MutableSet] with the given elements.
  * Elements of the set are iterated in the order they were specified.
+ * @sample samples.collections.Collections.Sets.mutableSet
  */
 public fun <T> mutableSetOf(vararg elements: T): MutableSet<T> = elements.toCollection(LinkedHashSet(mapCapacity(elements.size)))