Update Dispatchers docs to account for native-mt changes

Fixes #1828

Author: elizarov

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.
      */