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.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>