media: doc: pixfmt-yuv: Document subsampling in more details

Document YUV subsampling, including chroma spatial siting, and replace
the siting examples in individual formats by references to the common
documentation.

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>