Add paragraph about parent cancellation to async.

Mention structured concurrency, but do not explain it there as the term should be easily googlable

Fixes #787

Author: qwwdfsad

kotlinx-coroutines-core/common/src/Builders.common.kt

@@ -60,6 +60,9 @@ public fun CoroutineScope.launch(
 /**
  * Creates a coroutine and returns its future result as an implementation of [Deferred].
  * The running coroutine is cancelled when the resulting deferred is [cancelled][Job.cancel].
+ * The resulting coroutine has a key difference compared with similar primitives in other languages
+ * and frameworks: it cancels the parent job (or outer scope) on failure to enforce *structured concurrency* paradigm.
+ * To change that behaviour, supervising parent ([SupervisorJob] or [supervisorScope]) can be used.
  *
  * Coroutine context is inherited from a [CoroutineScope], additional context elements can be specified with [context] argument.
  * If the context does not have any dispatcher nor any other [ContinuationInterceptor], then [Dispatchers.Default] is used.
@@ -72,8 +75,6 @@ public fun CoroutineScope.launch(
  * the resulting [Deferred] is created in _new_ state. It can be explicitly started with [start][Job.start]
  * function and will be started implicitly on the first invocation of [join][Job.join], [await][Deferred.await] or [awaitAll].
  *
- * @param context additional to [CoroutineScope.coroutineContext] context of the coroutine.
- * @param start coroutine start option. The default value is [CoroutineStart.DEFAULT].
  * @param block the coroutine code.
  */
 public fun <T> CoroutineScope.async(