RISC-V: rework comments in ISA string parser

I have found these comments to not be at all helpful whenever I look at
the parser. Further, the comments in the default case (single letter
parser) are not quite right either.
Group the comments into a larger one at the start of each case, that
attempts to explain things at a higher level.

Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230607-headpiece-tannery-83ed5cc4856a@spud
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>