V4L/DVB (10980): doc: improve the v4l2-framework documentation.

Emphasize the need to call i2c_set_adapdata and clarify the use of the
chipid in v4l2_i2c_new_(probed_)device().

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>