sch_cake: revise Diffserv docs

sch_cake: revise Diffserv docs

Documentation incorrectly stated that CS1 is equivalent to LE for
diffserv8. But when LE was added to the table, CS1 was pushed into tin
1, leaving only LE in tin 0.

Also "TOS1" no longer exists, as that is the same codepoint as LE.

Make other tweaks properly distinguishing codepoints from classes and
putting current Diffserve codepoints ahead of legacy ones.

Signed-off-by: Kevin Bracey <kevin@bracey.fi>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://lore.kernel.org/r/20220106215637.3132391-1-kevin@bracey.fi
Signed-off-by: Jakub Kicinski <kuba@kernel.org>