Clarify ReentrantReadWriteLock.write behavior regarding upgrade from read to write lock

#KT-17704 Fixed

libraries/stdlib/src/kotlin/concurrent/Locks.kt

@@ -1,3 +1,19 @@
+/*
+ * Copyright 2010-2017 JetBrains s.r.o.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
 @file:JvmVersion
 @file:JvmName("LocksKt")
 package kotlin.concurrent
@@ -37,8 +53,15 @@ public inline fun <T> ReentrantReadWriteLock.read(action: () -> T): T {
 
 /**
  * Executes the given [action] under the write lock of this lock.
- * The method does upgrade from read to write lock if needed.
- * If such write has been initiated by checking some condition, the condition must be rechecked inside the action to avoid possible races.
+ *
+ * The function does upgrade from read to write lock if needed, but this upgrade is not atomic
+ * as such upgrade is not supported by [ReentrantReadWriteLock].
+ * In order to do such upgrade this function first releases all read locks held by this thread,
+ * then acquires write lock, and after releasing it acquires read locks back again.
+ *
+ * Therefore if the [action] inside write lock has been initiated by checking some condition,
+ * the condition must be rechecked inside the [action] to avoid possible races.
+ *
  * @return the return value of the action.
  */
 @kotlin.internal.InlineOnly