f2fs: update f2fs document regarding to fsync_mode

This patch adds missing fsync_mode entry in f2fs document.

Change-Id: I77f66131cb6ce9be09195296062824b9454f1ba3
Fixes: 04485987f053 ("f2fs: introduce async IPU policy")
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
2 files changed