Reference not updated to describe disjoint capture in closures

[pali6]

The Capture Modes section of the Rust reference seems to predate the implementation of the capture disjoint fields RFS and says that composite types are always captured entirely which is no longer the case.

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@rustbot modify labels: A-docs

[pali6]

Whoops, I guess I should have made this in https://github.com/rust-lang/reference instead. Apologies, I was not aware it's a separate repo.

[ehuss]

Thanks for the report! There is open work at rust-lang/reference#1059 to add the new information. Closing as a duplicate of rust-lang/reference#1066.