message: Add some clarity on the license/license-file warning.
diff: @@ -1398,8 +1398,12 @@ impl TomlManifest {
  );
  if project.license_file.is_some() && project.license.is_some() {
  manifest.warnings_mut().add_warning(
  - "only one of `license` or \
  - `license-file` is necessary"
  + "only one of `license` or `license-file` is necessary\n\
  + `license` should be used if the package license can be expressed \
  + with a standard SPDX expression.\n\
  + `license-file` should be used if the package uses a non-standard license.\n\
  + See https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields \
  + for more information."
  .to_string(),
  );
  }
  