diff --git a/documentation/ref-manual/ref-variables.xml b/documentation/ref-manual/ref-variables.xml
index fb337913ca..67ea422618 100644
--- a/documentation/ref-manual/ref-variables.xml
+++ b/documentation/ref-manual/ref-variables.xml
@@ -2199,6 +2199,15 @@
                     you can choose, see
                     <link linkend='var-IMAGE_TYPES'><filename>IMAGE_TYPES</filename></link>.
                 </para>
+
+                <note>
+                    Due to the way this variable is processed, it is not
+                    possible to update its contents using
+                    <filename>_append</filename> or
+                    <filename>_prepend</filename>. To add one or more
+                    additional options to this variable the
+                    <filename>+=</filename> operator must be used.
+                </note>
             </glossdef>
         </glossentry>