RISC-V: clarify ISA string ordering rules in cpu.c

While the current list of rules may have been accurate when created
it now lacks some clarity in the face of isa-manual updates. Instead of
trying to continuously align this rule-set with the one in the
specifications, change the role of this comment.

This particular comment is important, as the array it "decorates"
defines the order in which the ISA string appears to userspace in
/proc/cpuinfo.

Re-jig and strengthen the wording to provide contributors with a set
order in which to add entries & note why this particular struct needs
more attention than others.

While in the area, add some whitespace and tweak some wording for
readability's sake.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221205144525.2148448-2-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>