MDL-44764 atto_image: Reword the alt related labels

lib/editor/atto/plugins/image/lang/en/atto_image.php

@@ -35,11 +35,11 @@
 $string['alignment_top'] = 'Top';
 $string['browserepositories'] = 'Browse repositories...';
 $string['createimage'] = 'Insert image';
-$string['enteralt'] = 'Enter alternative text';
+$string['enteralt'] = 'Describe this image for someone who cannot see it';
 $string['enterurl'] = 'Enter URL';
 $string['height'] = 'Height';
-$string['presentation'] = 'Decorative only';
+$string['presentation'] = 'Description not necessary';
 $string['pluginname'] = 'Image';
-$string['presentationoraltrequired'] = 'Images must be marked as decorative only, or have suitable alternative text.';
+$string['presentationoraltrequired'] = 'Images must have a description, except if the description is marked as not necessary.';
 $string['preview'] = 'Preview';
 $string['width'] = 'Width';