Auto merge of #2889 - bennofs:fix-cargo-rustc-help, r=alexcrichton
Auto merge of #2889 - bennofs:fix-cargo-rustc-help, r=alexcrichton

fix help for cargo rustc -p

cargo rustc -p does the same as cargo build -p, so I just copied it from there.

eminence/cargo,tshepang/cargo,vojtechkral/cargo,achanda/cargo,brson/cargo,brson/cargo,tsurai/cargo,eminence/cargo,tsurai/cargo,rustyhorde/cargo,brson/cargo,durka/cargo,tshepang/cargo,achanda/cargo,rust-lang/cargo,achanda/cargo,tsurai/cargo,rust-lang/cargo,tshepang/cargo,nipunn1313/cargo,eminence/cargo,durka/cargo,rustyhorde/cargo,durka/cargo,rustyhorde/cargo,brson/cargo,tshepang/cargo,vojtechkral/cargo,AndiDog/cargo,nipunn1313/cargo,rust-lang/cargo,eminence/cargo,tsurai/cargo,rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo,brson/cargo,AndiDog/cargo,AndiDog/cargo,AndiDog/cargo,vojtechkral/cargo,sbeckeriv/cargo,durka/cargo,sbeckeriv/cargo,vojtechkral/cargo,achanda/cargo,rust-lang/cargo,nipunn1313/cargo,sbeckeriv/cargo,AndiDog/cargo,sbeckeriv/cargo