docs: v4l2codecs: Add plugin index documentation

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5555>

subprojects/gst-plugins-bad/sys/v4l2codecs/plugin.c

@@ -18,6 +18,28 @@
  * Boston, MA 02110-1301, USA.
  */
 
+/**
+ * plugin-v4l2codecs:
+ * @title: V4L2 Stateless codec
+ * @short_description: V4L2 plugin to support Statless codec drivers
+ *
+ * This plugin provides support for Video4Linux2 drivers implementing stateless
+ * video codecs. The plugin will generate elements based on the system
+ * capabilities. For this reason, this documentation may differ from output
+ * of running `gst-inspect-1.0` on your target.
+ *
+ * If you are having issues getting any elementis to be registered, you may want
+ * to verify that your user have adequate permissions to access media and video
+ * devices. These Linux devices are usually found in `/dev/media*` and
+ * `/dev/video*`.
+ *
+ * This documentation as been generated with the use of the environment variable
+ * `GST_V4L2_CODEC_GEN_DOC=1`. Using tis environment outside of the documentation
+ * generation will render your codecs unusable.
+ *
+ * Since: 1.18
+ */
+
 #ifdef HAVE_CONFIG_H
 #include "config.h"
 #endif