dspace-api: comment ImageMagick filters

Add some comments to document the functionality of the ImageMagick
thumbnail filters. This will help others understand it later when
we need to re-factor them.

dspace-api/src/main/java/org/dspace/app/mediafilter/ImageMagickPdfThumbnailFilter.java

@@ -22,7 +22,9 @@ public class ImageMagickPdfThumbnailFilter extends ImageMagickThumbnailFilter {
         File f2 = null;
         File f3 = null;
         try {
+            // Step 1: get an image from our PDF file, with PDF-specific processing options
             f2 = getImageFile(f, verbose);
+            // Step 2: use the image above to create the final resized and rotated thumbnail
             f3 = getThumbnailFile(f2, verbose);
             byte[] bytes = Files.readAllBytes(f3.toPath());
             return new ByteArrayInputStream(bytes);

dspace-api/src/main/java/org/dspace/app/mediafilter/ImageMagickThumbnailFilter.java

@@ -116,6 +116,13 @@ public abstract class ImageMagickThumbnailFilter extends MediaFilter {
         return f2;
     }
 
+    /**
+     * Return an image from a bitstream with specific processing options for
+     * PDFs. This is only used by ImageMagickPdfThumbnailFilter in order to
+     * generate an intermediate image file for use with getThumbnailFile. It
+     * is unfortunate that this means we convert from PDF to JPEG to JPEG,
+     * which incurs generation loss.
+     */
     public File getImageFile(File f, boolean verbose)
         throws IOException, InterruptedException, IM4JavaException {
         File f2 = new File(f.getParentFile(), f.getName() + ".jpg");