[media] Documentation: Fix V4L2_CTRL_FLAG_VOLATILE

V4L2_CTRL_FLAG_VOLATILE behaviour when V4L2_CTRL_FLAG_EXECUTE_ON_WRITE
is set was not properly explained.

Also set some hyperlink to ease the Documentation browsing.

Credit-to: Hans Verkuil <hansverk@cisco.com>

[mchehab@s-opensource.com: fix a trivial merge conflict]
Reported-by: Dimitrios Katsaros <patcherwork@gmail.com>
Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>