Improve XPM load failure message (bug#33126)

* src/image.c (xpm_load_image): Only XPM3 is supported, so make that
explicit.

etc/PROBLEMS

@@ -2616,6 +2616,13 @@ please call support for your X-server and see if you can get a fix.
 If you do, please send it to bug-gnu-emacs@gnu.org so we can list it here.
 
 
+* Runtime problems specific to macOS
+
+** macOS doesn't come with libxpm, so only XPM3 is supported.
+
+Libxpm is available for macOS as part of the XQuartz project.
+
+
 * Build-time problems
 
 ** Configuration

src/image.c

@@ -4308,7 +4308,7 @@ xpm_load_image (struct frame *f,
   return 1;
 
  failure:
-  image_error ("Invalid XPM file (%s)", img->spec);
+  image_error ("Invalid XPM3 file (%s)", img->spec);
   x_destroy_x_image (ximg);
   x_destroy_x_image (mask_img);
   x_clear_image (f, img);