2021: Update for disjoint captures (RFC 2229)

The closure type page needs to be updated for the 2021 changes. See [RFC 2229](https://github.com/rust-lang/rfcs/blob/master/text/2229-capture-disjoint-fields.md) and [edition guide chapter](https://github.com/rust-lang/edition-guide/blob/master/src/rust-2021/disjoint-capture-in-closures.md).

(Note: PR #1059 is already open to resolve this.)
