(Other Image Types): Add description of :foreground

and :background properties of mono PBM images.

lispref/display.texi

@@ -2648,7 +2648,20 @@ See the @file{etc/NEWS} file to make sure.
 @cindex PBM
 
   For PBM images, specify image type @code{pbm}.  Color, gray-scale and
-monochromatic images are supported.
+monochromatic images are supported.   For mono PBM images, two additional
+image properties are supported.
+
+@table @code
+@item :foreground @var{foreground}
+The value, @var{foreground}, should be a string specifying the image
+foreground color.  This color is used for each pixel in the XBM that is
+1.  The default is the frame's foreground color.
+
+@item :background @var{background}
+The value, @var{background}, should be a string specifying the image
+background color.  This color is used for each pixel in the XBM that is
+0.  The default is the frame's background color.
+@end table
 
   For JPEG images, specify image type @code{jpeg}.