One more improvement of left/right-fringe display spec docs

* doc/lispref/display.texi (Fringe Bitmaps): Yet another
clarification of how to use FACE in left/right-fringe display
spec.

doc/lispref/display.texi

@@ -4282,7 +4282,11 @@ background colors are to be used to display the bitmap, using the
 attributes of the @code{fringe} face for colors that @var{face} didn't
 specify.  If @var{face} is omitted, that means to use the attributes
 of the @code{default} face for the colors which the @code{fringe} face
-didn't specify.
+didn't specify.  For predictable results that don't depend on the
+attributes of the @code{default} and @code{fringe} faces, we recommend
+you never omit @var{face}, but always provide a specific face.  In
+particular, if you want the bitmap to be always displayed in the
+@code{fringe} face, use @code{fringe} as @var{face}.
 
   For instance, to display an arrow in the left fringe, using the
 @code{warning} face, you could say something like: