riscv: Improve description for RISCV_ISA_SVPBMT Kconfig symbol

This improves the symbol's description to make it easier for
people to understand what it is about.

Suggested-by: Christoph Hellwig <hch@lst.de>
Suggested-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20220526205646.258337-3-heiko@sntech.de
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>