[gtk/font-chooser-docs] fontchooser: Add some details to the docs

[Matthias Clasen <matthiasc src gnome org>]

commit b894c66101fee8e53d2a847e619d0924bd67284c
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Sep 21 07:28:26 2022 -0400

    fontchooser: Add some details to the docs

 gtk/gtkfontchooser.c | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/gtk/gtkfontchooser.c b/gtk/gtkfontchooser.c
index fb87ed0ca4..39b8fa88b3 100644
--- a/gtk/gtkfontchooser.c
+++ b/gtk/gtkfontchooser.c
@@ -524,6 +524,10 @@ gtk_font_chooser_get_level (GtkFontChooser *fontchooser)
  *
  * Gets the currently-selected font features.
  *
+ * The format of the returned string is compatible with the
+ * [CSS font-feature-settings property](https://www.w3.org/TR/css-fonts-4/#font-rend-desc).
+ * It can be passed to [func PangoAttrFontFeatures new].
+ *
  * Returns: the currently selected font features
  */
 char *