alpha: Add documentation
diff --git a/gst/alpha/gstalpha.c b/gst/alpha/gstalpha.c
index a88a938..fd189d3 100644
--- a/gst/alpha/gstalpha.c
+++ b/gst/alpha/gstalpha.c
@@ -20,6 +20,25 @@
* Boston, MA 02111-1307, USA.
*/
+/**
+ * SECTION:element-alpha
+ *
+ * The alpha element adds an alpha channel to a video stream. The values
+ * of the alpha channel can be either be set to a constant or can be
+ * dynamically calculated via chroma keying, e.g. blue can be set as
+ * the transparent color.
+ *
+ * Sample pipeline:
+ * |[
+ * gst-launch videotestsrc pattern=smpte75 ! alpha method=green ! \
+ * videomixer name=mixer ! ffmpegcolorspace ! autovideosink \
+ * videotestsrc pattern=snow ! mixer.
+ * ]| This pipeline adds a alpha channel to the SMPTE color bars
+ * with green as the transparent color and mixes the output with
+ * a snow video stream.
+ */
+
+
#ifdef HAVE_CONFIG_H
#include "config.h"
#endif
