Add comments for ItemAdded/Removed (suggestion from peppy)

osu.Framework/Audio/AudioCollectionManager.cs

@@ -55,10 +55,23 @@ protected override void UpdateChildren()
             }
         }
 
+        /// <summary>
+        /// An item has been added to <see cref="Items"/>.
+        /// </summary>
+        /// <param name="item">The item which was added.</param>
         protected virtual void ItemAdded(T item)
         {
         }
 
+        /// <summary>
+        /// An item has been removed from <see cref="Items"/>.
+        /// Importantly, dispose of the <paramref name="item"/> here if required.
+        /// </summary>
+        /// <remarks>
+        /// Disposal is left up to the implementor because some cases of removal see items managed by a higher
+        /// level class, i.e. SampleBassFactory.
+        /// </remarks>
+        /// <param name="item">The item which was removed.</param>
         protected virtual void ItemRemoved(T item)
         {
         }