[media] doc: v4l: selection: choose pixels as units for selection rectangles

Pixels were preferred units for selection rectangles over driver-dependent
units for almost all use cases. Therefore the units were fixed to pixels.

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>