Auto merge of #6516 - ehuss:doc-fix-overview, r=alexcrichton
Auto merge of #6516 - ehuss:doc-fix-overview, r=alexcrichton

Add a high-level overview of how `fix` works.

The fix code is pretty well commented, but I think it would be helpful to have an overview of how the pieces fit together.

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