Auto merge of #5081 - matklad:document-not-all-the-things, r=alexcrichton
Auto merge of #5081 - matklad:document-not-all-the-things, r=alexcrichton

Support --exclude option for `cargo doc`

I think this should have been implemented when the feature was added for
other commands. Probably just an oversight.

cc https://github.com/rust-lang/cargo/pull/4031

r? @alexcrichton

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