docs: Clean up the ToggleButton description

- Use different headings
- Fix the coding style of the example

gtk/gtktogglebutton.c

@@ -19,7 +19,7 @@
  * Modified by the GTK+ Team and others 1997-2000.  See the AUTHORS
  * file for a list of people on the GTK+ Team.  See the ChangeLog
  * files for a list of changes.  These files are distributed with
- * GTK+ at ftp://ftp.gtk.org/pub/gtk/. 
+ * GTK+ at ftp://ftp.gtk.org/pub/gtk/.
  */
 
 #include "config.h"
@@ -55,7 +55,7 @@
  * To simply switch the state of a toggle button, use
  * [method@Gtk.ToggleButton.toggled].
  *
- * # Grouping
+ * ## Grouping
  *
  * Toggle buttons can be grouped together, to form mutually exclusive
  * groups - only one of the buttons can be toggled at a time, and toggling
@@ -63,20 +63,25 @@
  *
  * To add a `GtkToggleButton` to a group, use [method@Gtk.ToggleButton.set_group].
  *
- * # CSS nodes
+ * ## CSS nodes
  *
  * `GtkToggleButton` has a single CSS node with name button. To differentiate
- * it from a plain `GtkButton`, it gets the .toggle style class.
+ * it from a plain `GtkButton`, it gets the `.toggle` style class.
  *
  * ## Creating two `GtkToggleButton` widgets.
  *
  * ```c
- * static void output_state (GtkToggleButton *source, gpointer user_data)
+ * static void
+ * output_state (GtkToggleButton *source,
+ *               gpointer         user_data)
  * {
- *   printf ("Active: %d\n", gtk_toggle_button_get_active (source));
+ *   g_print ("Toggle button "%s" is active: %s",
+ *            gtk_button_get_label (GTK_BUTTON (source)),
+ *            gtk_toggle_button_get_active (source) ? "Yes" : "No");
  * }
  *
- * void make_toggles (void)
+ * static void
+ * make_toggles (void)
  * {
  *   GtkWidget *window, *toggle1, *toggle2;
  *   GtkWidget *box;
@@ -85,7 +90,7 @@
  *   window = gtk_window_new ();
  *   box = gtk_box_new (GTK_ORIENTATION_VERTICAL, 12);
  *
- *   text = "Hi, I’m a toggle button.";
+ *   text = "Hi, I’m toggle button one";
  *   toggle1 = gtk_toggle_button_new_with_label (text);
  *
  *   g_signal_connect (toggle1, "toggled",
@@ -93,7 +98,7 @@
  *                     NULL);
  *   gtk_box_append (GTK_BOX (box), toggle1);
  *
- *   text = "Hi, I’m a toggle button.";
+ *   text = "Hi, I’m toggle button two";
  *   toggle2 = gtk_toggle_button_new_with_label (text);
  *   g_signal_connect (toggle2, "toggled",
  *                     G_CALLBACK (output_state),