[gtk/popup-shadow-width] Update popover css docs

[Matthias Clasen <matthiasc src gnome org>]

commit 7ebc3a9489a55191a1d4f16296d75ffefdfa19c8
Author: Matthias Clasen <mclasen redhat com>
Date:   Sun Feb 14 11:14:23 2021 -0500

    Update popover css docs
    
    Mention that shadows are now possible.

 gtk/gtkpopover.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)
---
diff --git a/gtk/gtkpopover.c b/gtk/gtkpopover.c
index bf9be24709..b00cdd3827 100644
--- a/gtk/gtkpopover.c
+++ b/gtk/gtkpopover.c
@@ -83,14 +83,15 @@
  * to differentiate from plain popovers.
  *
  * When styling a popover directly, the popover node should usually
- * not have any background.
+ * not have any background. The visible part of the popover can have a shadow.
+ * To specify it in CSS, set the box-shadow of the contents node.
  *
  * Note that, in order to accomplish appropriate arrow visuals, #GtkPopover uses
  * custom drawing for the arrow node. This makes it possible for the arrow to
  * change its shape dynamically, but it also limits the possibilities of styling
  * it using CSS. In particular, the arrow gets drawn over the content node's
- * border so they look like one shape, which means that the border-width of
- * the content node and the arrow node should be the same. The arrow also does
+ * shadow border so they look like one shape, which means that the border width
+ * of the content node and the arrow node should be the same. The arrow also does
  * not support any border shape other than solid, no border-radius, only one
  * border width (border-bottom-width is used) and no box-shadow.
  */