fscrypt: add additional documentation for SM4 support

Add a paragraph about SM4, like there is for the other modes.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Tianjia Zhang <tianjia.zhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221201191452.6557-1-ebiggers@kernel.org