[rygel] docs: RygelDataSource: Mention adding API for transcoders.

[Murray Cumming <murrayc src gnome org>]

commit 7a6c49f15b7b84b046b1641247f38632a8afae1c
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Jan 7 11:26:02 2013 +0100

    docs: RygelDataSource: Mention adding API for transcoders.

 src/librygel-server/rygel-data-source.vala |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)
---
diff --git a/src/librygel-server/rygel-data-source.vala b/src/librygel-server/rygel-data-source.vala
index 07dfdef..3d70998 100644
--- a/src/librygel-server/rygel-data-source.vala
+++ b/src/librygel-server/rygel-data-source.vala
@@ -36,6 +36,17 @@ public errordomain Rygel.DataSourceError {
  *
  *  # It should support at least the file:/''''/ URI scheme.
  *  # It should be able to stream any kind of binary data, regardless of the format.
+ *
+ * RygelDataSource instances are provided by
+ * rygel_media_engine_create_data_source() which will return a derived #RygelDataSource
+ * that uses a specific IO backend and/or multimedia backend to stream the
+ * multimedia data at the URI.
+ *
+ * The derived RygelDataSource may provide additional API for use by
+ * the derived media engine. For instance, rygel_media_engine_get_transcoders()
+ * may return derived #RygelTranscoder instances, and these may use that
+ * additional API in their create_source() implementations, for instance
+ * to access resources or data structures of the specific multimedia backend.
  */
 public interface Rygel.DataSource : GLib.Object {
     /**