Add `man 3 mdast` to `mdast --help`
Add `man 3 mdast` to `mdast --help`

tanzania82/remarks,chcokr/mdast,ulrikaugustsson/mdast,eush77/remark,yukkurisinai/mdast,yukkurisinai/mdast,wooorm/remark,eush77/remark,ulrikaugustsson/mdast,chcokr/mdast