Auto merge of #8052 - ehuss:rustdoc-warnings, r=alexcrichton
Auto merge of #8052 - ehuss:rustdoc-warnings, r=alexcrichton

Squelch some rustdoc warnings.

Removes the warning `warning: `[target]` cannot be resolved, ignoring it.` when documenting cargo itself.

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