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
IMAGE_TYPES.
+
+
+ Due to the way this variable is processed, it is not
+ possible to update its contents using
+ _append or
+ _prepend. To add one or more
+ additional options to this variable the
+ += operator must be used.
+