stub: Document StreamObserver is an async API

Missing docs were brought up in grpc#8423

stub/src/main/java/io/grpc/stub/StreamObserver.java

@@ -31,6 +31,11 @@
  * not need to be synchronized together; incoming and outgoing directions are independent.
  * Since individual {@code StreamObserver}s are not thread-safe, if multiple threads will be
  * writing to a {@code StreamObserver} concurrently, the application must synchronize calls.
+ *
+ * <p>This API is asynchronous, so methods may return before the operation completes. gRPC
+ * guarantees it does not block on I/O in its implementation, but applications are allowed to
+ * perform blocking operations in their implementations. However, doing so will delay other
+ * callbacks because the methods cannot be called concurrently.
  */
 public interface StreamObserver<V>  {
   /**