Auto merge of #6898 - ehuss:doc-auto-discovery, r=alexcrichton
Auto merge of #6898 - ehuss:doc-auto-discovery, r=alexcrichton

Update documentation for auto-discovery.

The old documentation was slightly wrong, and didn't really go into much detail.
This adds some elaboration.

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