gio: Document thread safety of the streams API

Specifically, GIOStream and the TLS connection streams.

Includes wording adapted from suggestions by Dan Winship
<danw@gnome.org>.

https://bugzilla.gnome.org/show_bug.cgi?id=735754

gio/goutputstream.c

@@ -40,7 +40,10 @@
  * (g_output_stream_flush()). 
  *
  * To copy the content of an input stream to an output stream without 
- * manually handling the reads and writes, use g_output_stream_splice(). 
+ * manually handling the reads and writes, use g_output_stream_splice().
+ *
+ * See the documentation for #GIOStream for details of thread safety of
+ * streaming APIs.
  *
  * All of these functions have async variants too.
  **/