Document `FT_Face_Rec_::box' with variable fonts (#54969).

As pointed out in the "Note:" in the section of

  ... the xMin, yMin, xMax, yMax, macStyle and lowestRecPPEM fields in
  the font header ('head') table are not supported by variation data and
  should only be used in relation to the default instance for the font.

Document that `FT_Face_Rec_::box' does not vary with OpenType variable
1 file changed