RISC-V: clarify the QEMU workaround in ISA parser

Extensions prefixed with "Su" won't corrupt the workaround in many
cases.  The only exception is when the first multi-letter extension in the
ISA string begins with "Su" and is not prefixed with an underscore.

For instance, following ISA string can confuse this QEMU workaround.

*   "rv64imacsuclic" (RV64I + M + A + C + "Suclic")

However, this case is very unlikely because extensions prefixed by either
"Z", "Sm" or "Ss" will most likely precede first.

For instance, the "Suclic" extension (draft as of now) will be placed after
related "Smclic" and "Ssclic" extensions.  It's also highly likely that
other unprivileged extensions like "Zba" will precede.

It's also possible to suppress the issue in the QEMU workaround with an
underscore.  Following ISA string won't confuse the QEMU workaround.

*   "rv64imac_suclic" (RV64I + M + A + C + delimited "Suclic")

This fix is to tell kernel developers the nature of this workaround
precisely.  There are some "Su*" extensions to be ratified but don't worry
about this workaround too much.

This commit comes with other minor editorial fixes (for minor wording and
spacing issues, without changing the meaning).

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/8a127608cf6194a6d288289f2520bd1744b81437.1690350252.git.research_trasio@irq.a4lg.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>