Document limitation of String.format

The null case is a bit special.

Author: romainbrenguier

src/main/java/java/lang/String.java

@@ -3986,6 +3986,11 @@ static String cproverFormatArgument(Object obj) {
      * <li>
      *   having 5 arguments or more makes the solver slow
      * </li>
+     * <li>
+     *   the string "null" is interpreted the same way {@code null} would be by
+     *   the JDK String.format function, which is correct for the %s format
+     *   specifier but not %b for instance.
+     * </li>
      * </ul>
      * @diffblue.untested
      */