message: Add missing doccomment for `SubstringSpec::new()`
diff: @@ -17,6 +17,7 @@ struct SubstringSpec {
  }
  impl SubstringSpec {
  + /// Constructs a new StringSpec with the given start index and length.
  fn new(start_index: usize, length: usize) -> Self {
  Self {
  start_index,
  