Auto merge of #6109 - joshtriplett:cargo-build-help, r=alexcrichton
Auto merge of #6109 - joshtriplett:cargo-build-help, r=alexcrichton

cargo build: Simplify help for pkgids

Drop the full paragraph at the end of the help message in favor of a
reference to `cargo help pkdid`.

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