f2fs: add missing documents of reserve_root/resuid/resgid

Add missing documents.

Fixes: 7e65be49 ("f2fs: add reserved blocks for root user")
Fixes: 7c2e5963 ("f2fs: add resgid and resuid to reserve root blocks")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>