Clarify thread locals documentation

Fixes #985

Author: qwwdfsad

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

@@ -81,9 +81,11 @@ public interface ThreadContextElement<S> : CoroutineContext.Element {
  * Wraps [ThreadLocal] into [ThreadContextElement]. The resulting [ThreadContextElement]
  * maintains the given [value] of the given [ThreadLocal] for coroutine regardless of the actual thread its is resumed on.
  * By default [ThreadLocal.get] is used as a value for the thread-local variable, but it can be overridden with [value] parameter.
+ * Beware that context element **does not track** modifications of the thread-local and accessing thread-local from coroutine
+ * without corresponding context element returns **undefined** value. See the examples for a detailed description.
  *
- * Example usage looks like this:
  *
+ * Example usage:
  * ```
  * val myThreadLocal = ThreadLocal<String?>()
  * ...
@@ -97,7 +99,7 @@ public interface ThreadContextElement<S> : CoroutineContext.Element {
  * println(myThreadLocal.get()) // Prints "null"
  * ```
  *
- * Note that the context element does not track modifications of the thread-local variable, for example:
+ * The context element does not track modifications of the thread-local variable, for example:
  *
  * ```
  * myThreadLocal.set("main")
@@ -109,12 +111,27 @@ public interface ThreadContextElement<S> : CoroutineContext.Element {
  * ```
  *
  * Use `withContext` to update the corresponding thread-local variable to a different value, for example:
- *
  * ```
  * withContext(myThreadLocal.asContextElement("foo")) {
  *     println(myThreadLocal.get()) // Prints "foo"
  * }
  * ```
+ *
+ * Accessing thread-local without corresponding context element leads to undefined value:
+ * ```
+ * val tl = ThreadLocal.withInitial { "initial" }
+ *
+ * runBlocking {
+ *   println(tl.get()) // Will print "initial"
+ *   // Change context
+ *   withContext(tl.asContextElement("modified")) {
+ *     println(tl.get()) // Will print "modified"
+ *   }
+ *   // Context is changed again
+ *    println(tl.get()) // <- WARN: can print either "modified" or "initial"
+ * }
+ * ```
+ * to fix this behaviour use `runBlocking(tl.asContextElement())`
  */
 public fun <T> ThreadLocal<T>.asContextElement(value: T = get()): ThreadContextElement<T> =
     ThreadLocalElement(value, this)