message: Run doctest only when pnm is enabled
diff: @@ -31,7 +31,8 @@ use super::free_functions;
  /// source is some blob in memory and you have constructed the reader in another way. Here is an
  /// example with a `pnm` black-and-white subformat that encodes its pixel matrix with ascii values.
  ///
  -/// ```
  +#[cfg_attr(feature = "pnm", doc = "```")]
  +#[cfg_attr(not(feature = "pnm"), doc = "```no_run")]
  /// # use image::ImageError;
  /// # use image::io::Reader;
  /// # fn main() -> Result<(), ImageError> {
  