g-i: Add some nullable annotations

Based on grepping arg docs for NULL.
See https://gitlab.gnome.org/GNOME/pygobject/issues/261

gdk/x11/gdkcursor-x11.c

@@ -261,7 +261,7 @@ gdk_x11_cursor_create_for_name (GdkDisplay  *display,
 /**
  * gdk_x11_display_set_cursor_theme:
  * @display: (type GdkX11Display): a #GdkDisplay
- * @theme: the name of the cursor theme to use, or %NULL to unset
+ * @theme: (nullable): the name of the cursor theme to use, or %NULL to unset
  *         a previously set value
  * @size: the cursor size to use, or 0 to keep the previous size
  *

gdk/x11/gdkdisplay-x11.c

@@ -2822,8 +2822,8 @@ gdk_x11_display_error_trap_pop_ignored (GdkDisplay *display)
 
 /**
  * gdk_x11_set_sm_client_id:
- * @sm_client_id: the client id assigned by the session manager when the
- *    connection was opened, or %NULL to remove the property.
+ * @sm_client_id: (nullable): the client id assigned by the session manager
+ *    when the connection was opened, or %NULL to remove the property.
  *
  * Sets the `SM_CLIENT_ID` property on the application’s leader window so that
  * the window manager can save the application’s state using the X11R6 ICCCM

gtk/gtkactionable.c

@@ -144,7 +144,7 @@ gtk_actionable_get_action_target_value (GtkActionable *actionable)
 /**
  * gtk_actionable_set_action_target_value:
  * @actionable: a #GtkActionable widget
- * @target_value: a #GVariant to set as the target value, or %NULL
+ * @target_value: (nullable): a #GVariant to set as the target value, or %NULL
  *
  * Sets the target value of an actionable widget.
  *

gtk/gtkpagesetup.c

@@ -748,7 +748,7 @@ enum_to_string (GType type,
  * gtk_page_setup_to_key_file:
  * @setup: a #GtkPageSetup
  * @key_file: the #GKeyFile to save the page setup to
- * @group_name: the group to add the settings to in @key_file, 
+ * @group_name: (nullable): the group to add the settings to in @key_file,
  *      or %NULL to use the default name “Page Setup”
  * 
  * This function adds the page setup from @setup to @key_file.

gtk/gtkpapersize.c

@@ -911,7 +911,7 @@ gtk_paper_size_get_default_right_margin (GtkPaperSize *size,
 /**
  * gtk_paper_size_new_from_key_file:
  * @key_file: the #GKeyFile to retrieve the papersize from
- * @group_name: the name ofthe group in the key file to read,
+ * @group_name: (nullable): the name of the group in the key file to read,
  *     or %NULL to read the first group
  * @error: (allow-none): return location for an error, or %NULL
  *

gtk/gtkprintsettings.c

@@ -1766,7 +1766,7 @@ add_value_to_key_file (const gchar  *key,
  * gtk_print_settings_to_key_file:
  * @settings: a #GtkPrintSettings
  * @key_file: the #GKeyFile to save the print settings to
- * @group_name: the group to add the settings to in @key_file, or 
+ * @group_name: (nullable): the group to add the settings to in @key_file, or
  *     %NULL to use the default “Print Settings”
  *
  * This function adds the print settings from @settings to @key_file.

gtk/gtktreeselection.c

@@ -374,7 +374,7 @@ gtk_tree_selection_get_mode (GtkTreeSelection *selection)
 /**
  * gtk_tree_selection_set_select_function:
  * @selection: A #GtkTreeSelection.
- * @func: The selection function. May be %NULL
+ * @func: (nullable): The selection function. May be %NULL
  * @data: The selection function’s data. May be %NULL
  * @destroy: The destroy function for user data.  May be %NULL
  *

gtk/gtktreeview.c

@@ -11213,7 +11213,7 @@ gtk_tree_view_move_column_after (GtkTreeView       *tree_view,
 /**
  * gtk_tree_view_set_expander_column:
  * @tree_view: A #GtkTreeView
- * @column: %NULL, or the column to draw the expander arrow at.
+ * @column: (nullable): %NULL, or the column to draw the expander arrow at.
  *
  * Sets the column to draw the expander arrow at. It must be in @tree_view.  
  * If @column is %NULL, then the expander arrow is always at the first