Auto merge of #8767 - MonliH:master, r=ehuss
Auto merge of #8767 - MonliH:master, r=ehuss

Document RUSTFMT environment variable

This PR documents the `RUSTFMT` enviorment variable, as specified in [rust-lang/rustfmt/4426](https://github.com/rust-lang/rustfmt/issues/4426) and [rust-lang/rustfmt/4419](https://github.com/rust-lang/rustfmt/pull/4419).

eminence/cargo,tshepang/cargo,rust-lang/cargo,rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,eminence/cargo,tshepang/cargo,eminence/cargo,nipunn1313/cargo,rustyhorde/cargo,rustyhorde/cargo,rustyhorde/cargo,nipunn1313/cargo,tshepang/cargo,tshepang/cargo,eminence/cargo,nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo