Clarify write-body-to-stream docstring

Explicitly state that the output stream may be written to
asynchronously in the write-body-to-stream docstring. This allows for
asynchronous channels or streams to be returned as a response body.
Previously it may not have been obvious that this was possible
behavior.

Author: weavejester

ring-core/src/ring/core/protocols.clj

@@ -9,7 +9,8 @@
   "A protocol for writing data to the response body via an output stream."
   (write-body-to-stream [body response output-stream]
     "Write a value representing a response body to an output stream. The stream
-    will be closed after the value had been written."))
+    will be closed after the value had been written. The stream may be written
+    asynchronously."))
 
 (defn- ^Writer response-writer [response output-stream]
   (if-let [charset (response/get-charset response)]