[annotations] Add allow-none

This commit was created using a script that searched for all docstrings
containing a parameter and the string 'or %NULL'.
Gdk backends and demos excluded as they are not part of a public API

https://bugzilla.gnome.org/show_bug.cgi?id=610474

gtk/gtkinfobar.c

@@ -882,7 +882,7 @@ gtk_info_bar_new (void)
 
 /**
  * gtk_info_bar_new_with_buttons:
- * @first_button_text: stock ID or text to go in first button, or %NULL
+ * @first_button_text: (allow-none): stock ID or text to go in first button, or %NULL
  * @...: response ID for first button, then additional buttons, ending
  *    with %NULL
  *