[media] DocBook: add SDR specific info to G_MODULATOR / S_MODULATOR

Add SDR specific notes to G_MODULATOR / S_MODULATOR documentation.

Signed-off-by: Antti Palosaari <crope@iki.fi>
[hans.verkuil@cisco.com: add quotes in "The term 'modulator'"]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>

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