[Misc] Improve javadoc

xwiki-platform-core/xwiki-platform-mail/xwiki-platform-mail-send/xwiki-platform-mail-send-default/src/main/java/org/xwiki/mail/script/ScriptMimeMessage.java

@@ -86,7 +86,9 @@ public class ScriptMimeMessage extends ExtendedMimeMessage
      * @return the Mime Body Part object that was added. Returning it allows script to make modifications to that body
      *         part after it's been set (get/set some headers, etc)
      * @param mimeType the mime type of the content parameter
-     * @param content the content to include in the mail
+     * @param content the content to include in the mail. The type depends on the mimetype. For example for a mime
+     *        type of {@code text/plain} or {@code text/html}, the content should be a String, for a mime type of
+     *        {@code xwiki/template} it should be a {@link org.xwiki.model.reference.DocumentReference}, etc.
      */
     public BodyPart addPart(String mimeType, Object content)
     {
@@ -99,7 +101,9 @@ public BodyPart addPart(String mimeType, Object content)
      * @return the Mime Body Part object that was added. Returning it allows script to make modifications to that body
      *         part after it's been set (get/set some headers, etc)
      * @param mimeType the mime type of the content parameter
-     * @param content the content to include in the mail
+     * @param content the content to include in the mail. The type depends on the mimetype. For example for a mime
+     *        type of {@code text/plain} or {@code text/html}, the content should be a String, for a mime type of
+     *        {@code xwiki/template} it should be a {@link org.xwiki.model.reference.DocumentReference}, etc.
      * @param parameters the list of extra parameters. This is used for example to pass alternate content for the mail
      *        using the {@code alternate} key in the HTML Mime Body Part Factory. Mail headers can also be passed using
      *        the {@code headers} key with a {@code Map<String, String>} value containing header keys and values