documentation: Document PR_RISCV_SET_ICACHE_FLUSH_CTX prctl

Provide documentation that explains how to properly do CMODX in riscv.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240312-fencei-v13-3-4b6bdc2bbf32@rivosinc.com


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