media: doc: pixfmt-packed-yuv: Clarify naming scheme for 4:4:4 formats

Document the naming scheme for the existing packed YUV 4:4:4 formats, as
previously done for the RGB formats.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>