Improve MainCoroutineDispatcher.immediate documentation

Fixes #1219

Author: qwwdfsad

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

@@ -25,6 +25,9 @@ public abstract class MainCoroutineDispatcher : CoroutineDispatcher() {
      *   /*
      *    * If it is known that updateUiElement can be invoked both from the Main thread and from other threads,
      *    * `immediate` dispatcher is used as a performance optimization to avoid unnecessary dispatch.
+     *    *
+     *    * In that case, when `updateUiElement` is invoked from the Main thread, `uiElement.text` will be
+     *    * invoked immediately without any dispatching, otherwise, the `Dispatchers.Main` dispatch cycle will be triggered.
      *    */
      *   withContext(Dispatchers.Main.immediate) {
      *     uiElement.text = text

ui/kotlinx-coroutines-android/src/HandlerDispatcher.kt

@@ -34,6 +34,10 @@ public sealed class HandlerDispatcher : MainCoroutineDispatcher(), Delay {
      *   /*
      *    * If it is known that updateUiElement can be invoked both from the Main thread and from other threads,
      *    * `immediate` dispatcher is used as a performance optimization to avoid unnecessary dispatch.
+     *    *
+     *    * In that case, when `updateUiElement` is invoked from the Main thread, `uiElement.text` will be
+     *    * invoked immediately without any dispatching, otherwise, the `Dispatchers.Main` dispatch cycle via
+     *    * `Handler.post` will be triggered.
      *    */
      *   withContext(Dispatchers.Main.immediate) {
      *     uiElement.text = text