media: doc: pixfmt-rgb: Clarify naming scheme for RGB formats

The naming scheme for the RGB pixel formats has been developed
organically, and isn't consistent between formats using less than 8 bits
per pixels (mostly stored in 1 or 2 bytes per pixel, except for RGB666
that uses 4 bytes per pixel) and formats with 8 bits per pixel (stored
in 3 or 4 bytes). For the latter category, the names use a components
order convention that is the opposite of the first category, and the
opposite of DRM pixel formats. This has led to lots of confusion in the
past, and would really benefit from being explained more precisely. Do
so, which also prepares for the addition of additional RGB pixels
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>