f2fs: fix to document fault injection option and sysfs file

Commit 73faec4d9935 ("f2fs: add mount option to select fault injection
ratio") and Commit 087968974fcd ("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>
diff --git a/Documentation/filesystems/f2fs.txt b/Documentation/filesystems/f2fs.txt
index 4f6531a..8b04a63 100644
--- a/Documentation/filesystems/f2fs.txt
+++ b/Documentation/filesystems/f2fs.txt
@@ -155,6 +155,8 @@
                        enabled by default.
 data_flush             Enable data flushing before checkpoint in order to
                        persist data of regular and symlink.
+fault_injection=%d     Enable fault injection in all supported types with
+                       specified injection rate.
 mode=%s                Control block allocation mode which supports "adaptive"
                        and "lfs". In "lfs" mode, there should be no random
                        writes towards main area.