[gtk/gtk-3-24] FileChooserButton: Document the CSS nodes & class

[Daniel Boles <dboles src gnome org>]

commit c6b16fbbfa6b8e34fcac5b25bd22f99859b28fd1
Author: Daniel Boles <dboles src gmail com>
Date:   Thu Jun 20 20:51:13 2019 +0100

    FileChooserButton: Document the CSS nodes & class
    
    The only glancing mention of this we had was that GtkButton mentioned it
    in passing when discussing how `button` could get contextual classes.

 gtk/gtkfilechooserbutton.c | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/gtk/gtkfilechooserbutton.c b/gtk/gtkfilechooserbutton.c
index 19c1770154..6de8a4ee6d 100644
--- a/gtk/gtkfilechooserbutton.c
+++ b/gtk/gtkfilechooserbutton.c
@@ -96,6 +96,11 @@
  * > gtk_file_chooser_button_set_width_chars(), or pack the button in
  * > such a way that other interface elements give space to the
  * > widget.
+ *
+ * # CSS nodes
+ *
+ * GtkFileChooserButton has a CSS node with name “filechooserbutton”, containing
+ * a subnode for the internal button with name “button” and style class “.file”.
  */