Minor: add some clarifications

libraries/tools/kotlin-gradle-plugin/src/main/kotlin/org/jetbrains/kotlin/gradle/plugin/KotlinPlugin.kt

@@ -438,6 +438,14 @@ open class KotlinAndroidPlugin @Inject constructor(val scriptHandler: ScriptHand
             }
 
             javaTask.doFirst {
+                /*
+                 * It's important to modify javaTask.classpath only in doFirst,
+                 * because Android plugin uses ConventionMapping to modify it too (see JavaCompileConfigAction.execute),
+                 * and setting classpath explicitly prevents usage of Android mappings.
+                 * Also classpath setted by Android can be modified after excecution of some tasks (see VarianConfiguration.getCompileClasspath)
+                 * ex. it adds some support libraries jars after execution of prepareComAndroidSupportSupportV42311Library task,
+                 * so it's only safe to modify javaTask.classpath right before its usage
+                 */
                 javaTask.classpath += project.files(kotlinTask.property("kotlinDestinationDir"))
             }
         }