f2fs: fix to document fault injection option and sysfs file

Commit 73faec4d ("f2fs: add mount option to select fault injection
ratio") and Commit 08796897 ("f2fs: add fault injection to sysfs")
forget to document mount option and sysfs file.

This patch fixes to document them.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>