Mention exceptions in ThreadContextElement (see #773)

Author: qwwdfsad

core/kotlinx-coroutines-core/src/ThreadContextElement.kt

@@ -55,6 +55,8 @@ public interface ThreadContextElement<S> : CoroutineContext.Element {
      * This function is invoked before the coroutine in the specified [context] is resumed in the current thread
      * when the context of the coroutine this element.
      * The result of this function is the old value of the thread-local state that will be passed to [restoreThreadContext].
+     * This method should handle its own exceptions and do not rethrow it. Thrown exceptions will leave coroutine which
+     * context is updated in an undefined state.
      *
      * @param context the coroutine context.
      */
@@ -66,6 +68,8 @@ public interface ThreadContextElement<S> : CoroutineContext.Element {
      * if [updateThreadContext] was previously invoked on resume of this coroutine.
      * The value of [oldState] is the result of the previous invocation of [updateThreadContext] and it should
      * be restored in the thread-local state by this function.
+     * This method should handle its own exceptions and do not rethrow it. Thrown exceptions will leave coroutine which
+     * context is updated in an undefined state.
      *
      * @param context the coroutine context.
      * @param oldState the value returned by the previous invocation of [updateThreadContext].