f2fs: doc: fix description of max_small_discards

The description of max_small_discards is out-of-update in below two
aspects, fix it.
- it is disabled by default
- small discards will be issued during checkpoint

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

Author: chaseyu

Documentation/ABI/testing/sysfs-fs-f2fs

@@ -102,9 +102,9 @@ What:		/sys/fs/f2fs/<disk>/max_small_discards
 Date:		November 2013
 Contact:	"Jaegeuk Kim" <jaegeuk.kim@samsung.com>
 Description:	Controls the issue rate of discard commands that consist of small
-		blocks less than 2MB. The candidates to be discarded are cached until
-		checkpoint is triggered, and issued during the checkpoint.
-		By default, it is disabled with 0.
+		blocks less than 2MB. The candidates to be discarded are cached during
+		checkpoint, and issued by issue_discard thread after checkpoint.
+		It is enabled by default.
 
 What:		/sys/fs/f2fs/<disk>/max_ordered_discard
 Date:		October 2022