device: Refer the user to per-backend docs about devices.

The exact semantics of the device API can't be spelled out
out in the device API docs since the effects are so tied
to the specific backend.  To use a particular device in
practice the user should refer to the backend docs.

src/cairo-device.c

@@ -86,6 +86,12 @@
  *   cairo_device_release (device);
  * }
  * </programlisting></informalexample>
+ *
+ * <note><para>Please refer to the documentation of each backend for
+ * additional usage requirements, guarantees provided, and
+ * interactions with existing surface API of the device functions for
+ * surfaces of that type.
+ * </para></note>
  */
 
 static const cairo_device_t _nil_device = {