f2fs: update documents and a MAINTAINERS entry

This patch adds missing some description of sysfs entries in
 - Documentation/ABI/testing/sysfs-fs-f2fs
 - Documentation/filesystems/f2fs.txt.

And it adds a maintained document entry of F2FS in MAINTAINERS.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

Documentation/filesystems/f2fs.txt

@@ -190,6 +190,11 @@ Files in /sys/fs/f2fs/<devname>
                               of the filesystem utilization, and used by
                               F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
 
+ max_victim_search	      This parameter controls the number of trials to
+			      find a victim segment when conducting SSR and
+			      cleaning operations. The default value is 4096
+			      which covers 8GB block address range.
+
 ================================================================================
 USAGE
 ================================================================================