Clarified the standalone XML draft submission requirements, and mentioned the xml2rfc switch to use for v3 sources.

 - Legacy-Id: 17589

Author: levkowetz

ietf/templates/submit/upload_submission.html

@@ -44,10 +44,18 @@
       <h2>Upload an Internet-Draft</h2>
 
       <p>
-	 <i>Either a plain-text document or a valid .xml file which can be processed by the xml2rfc
-	 processor <b>must</b> be provided.<br/>
-	 If no .txt file is provided, one will be generated from the .xml file.<br/>
-	 Other file types are optional.</i>
+	 <i>
+            Either a plain-text document or a valid .xml file which can be processed by the xml2rfc
+            processor <b>must</b> be provided.
+            <br/>
+            For v3 .xml sources which include external files, this requires them to be run through xml2rfc with
+            the <tt>--prep<tt> option, to produce a completely expanded (standalone) .xml source file for
+            upload.
+            <br/>
+            If no .txt file is provided, one will be generated from the .xml file.
+            <br/>
+            Other file types are optional.
+         </i>
       </p>
 
       <form method="post" enctype="multipart/form-data">