Documentation does not explain details of linking binaries with native libraries

[kornelski]

Cargo documentation of build scripts doesn't say that the build script is only for the library target, and not (always) for binary targets.

Following conversation: https://users.rust-lang.org/t/does-cargo-run-a-build-script-for-bin-targets-too/3946/4

it'd be good to clarify that in documentation and recommend bin targets to link with the lib target from the same crate in order to take advantage of the build scripts.