Mark ui-backhandler as @ExperimentalComposeLibrary, and add comments

Author: Omico

gradle-plugins/compose/src/main/kotlin/org/jetbrains/compose/ComposePlugin.kt

@@ -61,6 +61,9 @@ abstract class ComposePlugin : Plugin<Project> {
         }
     }
 
+    // Please sort them in the order of stable, experimental, and deprecated,
+    // and keep them in alphabetical order.
+    // Thus make it easier to find a missing dependency.
     @Suppress("DEPRECATION")
     class Dependencies(project: Project) {
         val animation = composeDependency("org.jetbrains.compose.animation:animation")
@@ -78,11 +81,13 @@ abstract class ComposePlugin : Plugin<Project> {
         val runtime = composeDependency("org.jetbrains.compose.runtime:runtime")
         val runtimeSaveable = composeDependency("org.jetbrains.compose.runtime:runtime-saveable")
         val ui = composeDependency("org.jetbrains.compose.ui:ui")
-        val uiBackHandler = composeDependency("org.jetbrains.compose.ui:ui-backhandler")
         val uiTest = composeDependency("org.jetbrains.compose.ui:ui-test")
         val uiTooling = composeDependency("org.jetbrains.compose.ui:ui-tooling")
         val uiUtil = composeDependency("org.jetbrains.compose.ui:ui-util")
 
+        @ExperimentalComposeLibrary
+        val uiBackHandler = composeDependency("org.jetbrains.compose.ui:ui-backhandler")
+
         @Deprecated("Use desktop.uiTestJUnit4", replaceWith = ReplaceWith("desktop.uiTestJUnit4"))
         val uiTestJUnit4 = composeDependency("org.jetbrains.compose.ui:ui-test-junit4")