Rust 2021 documentation for: Disjoint capture in closures (RFC 2229)

[m-ou-se]

• Itemized summary
• Initial details section (from blog post)
• Full description including edge cases and migration instructions (if relevant)

https://github.com/rust-lang/edition-guide/blob/master/src/rust-2021/disjoint-capture-in-closures.md

[nikomatsakis]

cc @rust-lang/wg-rfc-2229 -- it'd be great to take some of the material from that blog post draft and open a PR into this file.

[nikomatsakis]

@rustbot assign @nikomatsakis

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[nikomatsakis]

@rustbot assign @roxelo