doc: document uv_async_(init|send) return values

Fixes: libuv#576
PR-URL: libuv#1435
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

docs/src/async.rst

@@ -35,12 +35,16 @@ API
 
     Initialize the handle. A NULL callback is allowed.
 
+    :returns: 0 on success, or an error code < 0 on failure.
+
     .. note::
         Unlike other handle initialization  functions, it immediately starts the handle.
 
 .. c:function:: int uv_async_send(uv_async_t* async)
 
-    Wakeup the event loop and call the async handle's callback.
+    Wake up the event loop and call the async handle's callback.
+
+    :returns: 0 on success, or an error code < 0 on failure.
 
     .. note::
         It's safe to call this function from any thread. The callback will be called on the