f2fs: add missing doc for fault injection sysfs

We supported configuring fault injection parameter via sysfs w/
below commits, however, we forgot to add doc entry, fix it.

commit 08796897 ("f2fs: add fault injection to sysfs")
/sys/fs/f2fs/fault_injection/fault_*

commit 1ecc0c5c ("f2fs: support configuring fault injection per superblock")
/sys/fs/f2fs/<device>/fault_*

Cc: Sheng Yong <shengyong@oppo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -669,3 +669,42 @@ Contact:	"Ping Xiong" <xiongping1@xiaomi.com>
 Description:	When DATA SEPARATION is on, it controls the age threshold to indicate
 		the data blocks as warm. By default it was initialized as 2621440 blocks
 		(equals to 10GB).
+
+What:		/sys/fs/f2fs/<disk>/fault_rate
+Date:		May 2016
+Contact:	"Sheng Yong" <shengyong@oppo.com>
+Contact:	"Chao Yu" <chao@kernel.org>
+Description:	Enable fault injection in all supported types with
+		specified injection rate.
+
+What:		/sys/fs/f2fs/<disk>/fault_type
+Date:		May 2016
+Contact:	"Sheng Yong" <shengyong@oppo.com>
+Contact:	"Chao Yu" <chao@kernel.org>
+Description:	Support configuring fault injection type, should be
+		enabled with fault_injection option, fault type value
+		is shown below, it supports single or combined type.
+
+		===================      ===========
+		Type_Name                Type_Value
+		===================      ===========
+		FAULT_KMALLOC            0x000000001
+		FAULT_KVMALLOC           0x000000002
+		FAULT_PAGE_ALLOC         0x000000004
+		FAULT_PAGE_GET           0x000000008
+		FAULT_ALLOC_BIO          0x000000010 (obsolete)
+		FAULT_ALLOC_NID          0x000000020
+		FAULT_ORPHAN             0x000000040
+		FAULT_BLOCK              0x000000080
+		FAULT_DIR_DEPTH          0x000000100
+		FAULT_EVICT_INODE        0x000000200
+		FAULT_TRUNCATE           0x000000400
+		FAULT_READ_IO            0x000000800
+		FAULT_CHECKPOINT         0x000001000
+		FAULT_DISCARD            0x000002000
+		FAULT_WRITE_IO           0x000004000
+		FAULT_SLAB_ALLOC         0x000008000
+		FAULT_DQUOT_INIT         0x000010000
+		FAULT_LOCK_OP            0x000020000
+		FAULT_BLKADDR            0x000040000
+		===================      ===========