[media] doc: v4l: add documentation for selection API

This patch adds a documentation for VIDIOC_{G/S}_SELECTION ioctl. Moreover, the
patch adds the description of modeling of composing, cropping and scaling
features in V4L2. Finally, some examples are presented.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
diff --git a/Documentation/DocBook/media/v4l/common.xml b/Documentation/DocBook/media/v4l/common.xml
index 9d39198..c79278a 100644
--- a/Documentation/DocBook/media/v4l/common.xml
+++ b/Documentation/DocBook/media/v4l/common.xml
@@ -1168,6 +1168,8 @@
     </section>
   </section>
 
+  &sub-selection-api;
+
   <section id="streaming-par">
     <title>Streaming Parameters</title>