PDFBOX-5660: add comment

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1923398 13f79535-47bb-0310-9956-ffa450edef68

Author: THausherr

pdfbox/src/main/java/org/apache/pdfbox/pdmodel/PDDocument.java

@@ -697,6 +697,8 @@ public void removePage(int pageNumber)
      */
     public PDPage importPage(PDPage page) throws IOException
     {
+        // BEWARE: when making changes here, make sure that these changes don't mess with the code
+        // in the splitter, and avoid making changes in the source document (as happened in PDFBOX-5809)
         PDPage importedPage = new PDPage(new COSDictionary(page.getCOSObject()), resourceCache);
         importedPage.getCOSObject().removeItem(COSName.PARENT);
         PDStream dest = new PDStream(this, page.getContents(), COSName.FLATE_DECODE);