Types section cleanup:

- Some rewording
- Modern PHP style prefers lowercase null/true/false
- Move contstants to constant section
So just because we didn't fight PSR-2 to death as we hoped we can ignore now we need to live with this silliness in the spec? Please don't lowercase this. Just look at the change before and after, "null type" vs `NULL` is much easier to discern "null type" vs `null`.

I won't repeat this on every single case but it applies.