Warn re. tiffs and jp2s in image file derivatives. (#921)

* Warn re. tiffs and jp2s in image file derivatives.

* Update wording.

modules/islandora_image/src/Plugin/Action/GenerateImageDerivativeFile.php

@@ -51,7 +51,7 @@ class GenerateImageDerivativeFile extends AbstractGenerateDerivativeMediaFile {
       '#title' => $this->t('Destination Image field'),
       '#default_value' => $this->configuration['destination_field_name'],
       '#description' => $this->t('This Action stores the derivative in an
-       Image field. If you need to store the result in a File field, use
+       Image field. If you are creating a TIFF or JP2, instead use
        "Generate a Derivative File for Media Attachment". Selected target field
        must be an additional field, not the media\'s main storage field.
        Selected target field must be present on the media.'),