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>