riscv: Clarify RISCV_ALTERNATIVE help text

Clarify RISCV_ALTERNATIVE's help text by pointing out that code
patching is not only done at boot time, but also module load time.
Also point out that this is the minimal possible overhead.
Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230224154601.88163-2-ajones@ventanamicro.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -379,8 +379,8 @@ config RISCV_ALTERNATIVE
 	help
 	  This Kconfig allows the kernel to automatically patch the
 	  errata required by the execution platform at run time. The
-	  code patching is performed once in the boot stages. It means
-	  that the overhead from this mechanism is just taken once.
+	  code patching overhead is minimal, as it's only done once
+	  at boot and once on each module load.
 
 config RISCV_ALTERNATIVE_EARLY
 	bool