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>
diff --git a/MAINTAINERS b/MAINTAINERS
index d5e4ff3..bd58e8e 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3629,6 +3629,7 @@
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git
 S:	Maintained
 F:	Documentation/filesystems/f2fs.txt
+F:	Documentation/ABI/testing/sysfs-fs-f2fs
 F:	fs/f2fs/
 F:	include/linux/f2fs_fs.h