Clarify possible values for BDF_PropertyRec.

include/ftbdf.h

@@ -4,7 +4,7 @@
 /*                                                                         */
 /*    FreeType API for accessing BDF-specific strings (specification).     */
 /*                                                                         */
-/*  Copyright 2002, 2003, 2004, 2006, 2009 by                              */
+/*  Copyright 2002-2004, 2006, 2009, 2014 by                               */
 /*  David Turner, Robert Wilhelm, and Werner Lemberg.                      */
 /*                                                                         */
 /*  This file is part of the FreeType project, and may only be used,       */
@@ -106,7 +106,8 @@ FT_BEGIN_HEADER
   *      The property type.
   *
   *    u.atom ::
-  *      The atom string, if type is @BDF_PROPERTY_TYPE_ATOM.
+  *      The atom string, if type is @BDF_PROPERTY_TYPE_ATOM.  May be
+  *      NULL, indicating an empty string.
   *
   *    u.integer ::
   *      A signed integer, if type is @BDF_PROPERTY_TYPE_INTEGER.