media: docs: selection: fix misleading sentence about the CROP API

The API limitation described here is about the CROP API, not about the
entire V4L2.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>