Document the error return values rather than calling them undefined.

Document the error return values rather than calling them undefined.

	* glib/gkeyfile.c (g_key_file_get_boolean)
	(g_key_file_get_boolean_list, g_key_file_get_integer)
	(g_key_file_get_integer_list, g_key_file_get_double)
	(g_key_file_get_double_list): Document the error return values
	rather than calling them undefined. #435885.

svn path=/trunk/; revision=5484