[media] Docbook: Fix documentation for media controller devnodes

The media-ctl userspace application assumes that all device nodes
are uniquelly defined via major,minor, just like v4l and fb.

That's ok for those types of devices, but, as we're adding support
for DVB at the API, what's written there at the DocBook is wrong.

So, fix it.

While here, fix the size of the reserved space inside the union,
with is 184, and not 180.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>