Clarify behavior of MutableSet.add (KT-29373)

runtime/src/main/kotlin/kotlin/collections/AbstractMutableSet.kt

@@ -17,7 +17,7 @@ public actual abstract class AbstractMutableSet<E> protected actual constructor(
     /**
      * Compares this set with another set instance with the unordered structural equality.
      *
-     * @return true, if [other] instance is a [Set] of the same size, all elements of which are contained in this set.
+     * @return `true`, if [other] instance is a [Set] of the same size, all elements of which are contained in this set.
      */
     override fun equals(other: Any?): Boolean {
         if (other === this) return true

runtime/src/main/kotlin/kotlin/collections/Set.kt

@@ -33,6 +33,12 @@ public interface MutableSet<E> : Set<E>, MutableCollection<E> {
     override fun iterator(): MutableIterator<E>
 
     // Modification Operations
+
+    /**
+     * Adds the specified element to the set.
+     *
+     * @return `true` if the element has been added, `false` if the element is already contained in the set.
+     */
     override fun add(element: E): Boolean
 
     override fun remove(element: E): Boolean