Auto merge of #6990 - notriddle:patch-1, r=ehuss
Auto merge of #6990 - notriddle:patch-1, r=ehuss

Add some hints to the docs for `cfg()` targets

https://users.rust-lang.org/t/what-cfg-expressions-are-expected-to-work-in-cargo-toml-quoted-targets/28662

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