[gtk] FileChooserButton: Document the CSS nodes & class

[Daniel Boles <dboles src gnome org>]

commit a03e531772ffe5168b5b27381ebba25310e57f9b
Author: Daniel Boles <dboles src gmail com>
Date:   Thu Jun 20 20:51:37 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,
    and even that's not relevant in master anymore... so drop it from there.

 gtk/gtkbutton.c            | 4 ++--
 gtk/gtkfilechooserbutton.c | 4 ++++
 2 files changed, 6 insertions(+), 2 deletions(-)
---
diff --git a/gtk/gtkbutton.c b/gtk/gtkbutton.c
index 3b701d9959..d428814dd1 100644
--- a/gtk/gtkbutton.c
+++ b/gtk/gtkbutton.c
@@ -46,8 +46,8 @@
  * can be made round by adding the .circular style class.
  *
  * Button-like widgets like #GtkToggleButton, #GtkMenuButton, #GtkVolumeButton,
- * #GtkLockButton, #GtkColorButton, #GtkFontButton or #GtkFileChooserButton use
- * style classes such as .toggle, .popup, .scale, .lock, .color, .file
+ * #GtkLockButton, #GtkColorButton or #GtkFontButton use style classes such as
+ * .toggle, .popup, .scale, .lock, .color on the button node
  * to differentiate themselves from a plain GtkButton.
  */
 
diff --git a/gtk/gtkfilechooserbutton.c b/gtk/gtkfilechooserbutton.c
index 73b6910299..2197e39615 100644
--- a/gtk/gtkfilechooserbutton.c
+++ b/gtk/gtkfilechooserbutton.c
@@ -96,6 +96,10 @@
  * > 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 single CSS node with the name “filechooserbutton”.
  */