Add documentation about multiple Cargo targets error

[volks73]

An error will occur if a Cargo project has multiple targets, i.e. library and one or more binaries defined in the package's manifest (Cargo.toml). Please note, "target" here is not the compiler target but the Cargo target. In this situation, the with_args must be used and the --lib or --bin <NAME> flag or option, respectively, must be added. Otherwise, an error message appears with the cargo rustc command that a single Cargo target must be called in order to pass rustc arguments.

[volks73]

This has been addressed with the new CargoRustcPrintCfg builder type and defaulting to adding the --lib flag. This avoids issues with multiple targets but if the user would like the compiler configuration for a specific Cargo target, the CargoRustcPrintCfg::cargo_target method is available in v0.2.0.