Update Dispatchers docs to account for native-mt changes

Fixes #1828

kotlinx-coroutines-core/common/src/Dispatchers.common.kt

@@ -15,8 +15,11 @@ public expect object Dispatchers {
  * [launch][CoroutineScope.launch], [async][CoroutineScope.async], etc
  * if neither a dispatcher nor any other [ContinuationInterceptor] is specified in their context.
  *
- * It is backed by a shared pool of threads on JVM. By default, the maximum number of threads used
- * by this dispatcher is equal to the number of CPU cores, but is at least two.
+ * Its implementation depends on the platform:
+ * - On Kotlin/JVM it is backed by a shared pool of threads. By default, the maximum number of threads used
+ * by this dispatcher is equal to the number of CPU cores, but is at least two.
+ * - On Kotlin/JS it backed by the JS event loop.
+ * - On Kotlin/Native it is backed by a single background thread that is created on the first use.
  */
  public val Default: CoroutineDispatcher
 
@@ -26,15 +29,17 @@ public expect object Dispatchers {
  *
  * Access to this property may throw an [IllegalStateException] if no main dispatchers are present in the classpath.
  *
- * Depending on platform and classpath it can be mapped to different dispatchers:
- * - On JS and Native it is equivalent to the [Default] dispatcher.
- * - On JVM it either the Android main thread dispatcher, JavaFx or Swing EDT dispatcher. It is chosen by the
+ * Depending on the platform and classpath it can be mapped to different dispatchers:
+ * - On Kotlin/JVM it either the Android main thread dispatcher, JavaFx or Swing EDT dispatcher. It is chosen by the
  * [`ServiceLoader`](https://docs.oracle.com/javase/8/docs/api/java/util/ServiceLoader.html).
+ * - On Kotlin/JS it is equivalent to the [Default] dispatcher.
+ * - On Kotlin/Native Apple platforms it maps to the Darwin main thread.
+ * - On other Kotlin/Native platforms it is equivalent to the [Default] dispatcher.
  *
- * In order to work with the `Main` dispatcher, the following artifact should be added to the project runtime dependencies:
+ * In order to work with the `Main` dispatcher on Kotlin/JVM, the following artifact should be added to the project runtime dependencies:
  * - `kotlinx-coroutines-android` — for Android Main thread dispatcher
  * - `kotlinx-coroutines-javafx` — for JavaFx Application thread dispatcher
- * - `kotlinx-coroutines-swing` — for Swing EDT dispatcher
+ * - `kotlinx-coroutines-swing` — for Swing EDT dispatcher.
  *
  * Implementation note: [MainCoroutineDispatcher.immediate] is not supported on the Native and JS platforms.
  */