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>

arch/riscv/Kconfig

@@ -364,8 +364,13 @@ config RISCV_ISA_SVPBMT
 	select RISCV_ALTERNATIVE
 	default y
 	help
-	   Adds support to dynamically detect the presence of the SVPBMT extension
-	   (Supervisor-mode: page-based memory types) and enable its usage.
+	   Adds support to dynamically detect the presence of the SVPBMT
+	   ISA-extension (Supervisor-mode: page-based memory types) and
+	   enable its usage.
+
+	   The memory type for a page contains a combination of attributes
+	   that indicate the cacheability, idempotency, and ordering
+	   properties for access to that page.
 
 	   The SVPBMT extension is only available on 64Bit cpus.