Document how to scale manually.

include/freetype/freetype.h

@@ -1530,7 +1530,13 @@ FT_BEGIN_HEADER
   /*    values of the corresponding fields in @FT_FaceRec.  Some values    */
   /*    like ascender or descender are rounded for historical reasons;     */
   /*    more precise values (for outline fonts) can be derived by scaling  */
-  /*    the corresponding @FT_FaceRec values manually.                     */
+  /*    the corresponding @FT_FaceRec values manually, with code similar   */
+  /*    to the following.                                                  */
+  /*                                                                       */
+  /*    {                                                                  */
+  /*      scaled_ascender = FT_MulFix( face->root.ascender,                */
+  /*                                   size_metrics->y_scale );            */
+  /*    }                                                                  */
   /*                                                                       */
   /*    Note that due to glyph hinting and the selected rendering mode     */
   /*    these values are usually not exact; consequently, they must be     */