[gtk] css: Add a comment

[Matthias Clasen <matthiasc src gnome org>]

commit c4b16f74245a86da930608c77316b0cc3d1cb0b7
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Feb 9 10:23:27 2020 -0500

    css: Add a comment
    
    All the other color values have this comment,
    best to be consistent.

 gtk/gtkcssstyleprivate.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/gtk/gtkcssstyleprivate.h b/gtk/gtkcssstyleprivate.h
index 3db7eb150b..1ee874d06b 100644
--- a/gtk/gtkcssstyleprivate.h
+++ b/gtk/gtkcssstyleprivate.h
@@ -137,7 +137,7 @@ struct _GtkCssOutlineValues {
   GtkCssValue *outline_style;
   GtkCssValue *outline_width;
   GtkCssValue *outline_offset;
-  GtkCssValue *outline_color;
+  GtkCssValue *outline_color; // NULL if currentColor
 };
 
 struct _GtkCssFontValues {