[gtk+] Adwaita: add a comment

[Lapo Calamandrei <lapo src gnome org>]

commit ebcf17debd0bcd781ef2d2d4202f8157d42046d9
Author: Lapo Calamandrei <calamandrei gmail com>
Date:   Tue May 9 13:38:03 2017 +0200

    Adwaita: add a comment
    
    Add a comment pointing to the related bug for dropdown menu
    margin.

 gtk/theme/Adwaita/_common.scss |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/gtk/theme/Adwaita/_common.scss b/gtk/theme/Adwaita/_common.scss
index d971ac7..904a02f 100644
--- a/gtk/theme/Adwaita/_common.scss
+++ b/gtk/theme/Adwaita/_common.scss
@@ -1898,7 +1898,7 @@ menubar,
 menu,
 .menu,
 .context-menu {
-  margin: 4px;
+  margin: 4px; // See https://bugzilla.gnome.org/show_bug.cgi?id=591258
   padding: 2px 0px;
   background-color: $menu_color;
   border: 1px solid $borders_color; // adds borders in a non composited env