docs: fully break lines in examples

Try to do a better job of keeping example content
from being too wide. It is often rendered as <pre>
text so the only time we can wrap it is in the source.

It is best to full break lines at all punctuation and
to try to keep the width under 70 chars or so.

gtk/gtkapplicationwindow.c

@@ -93,8 +93,11 @@
  *     "    </submenu>"
  *     "  </menu>"
  *     "</interface>");
+ *
+ * menubar = G_MENU_MODEL (gtk_builder_get_object (builder,
+ *                                                 "menubar"));
  * gtk_application_set_menubar (G_APPLICATION (app),
- *                              G_MENU_MODEL (gtk_builder_get_object (builder, "menubar")));
+ *                              menubar);
  * g_object_unref (builder);
  *
  * ...