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

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

  https://docs.microsoft.com/en-us/typography/opentype/spec/otvaroverview#variation-data-tables-and-miscellaneous-requirements

  ... 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
fonts.

include/freetype/freetype.h

@@ -958,6 +958,9 @@ FT_BEGIN_HEADER
    *     Note that the bounding box might be off by (at least) one pixel for
    *     hinted fonts.  See @FT_Size_Metrics for further discussion.
    *
+   *     Note that the bounding box does not vary in OpenType variable fonts
+   *     and should only be used in relation to the default instance.
+   *
    *   units_per_EM ::
    *     The number of font units per EM square for this face.  This is
    *     typically 2048 for TrueType fonts, and 1000 for Type~1 fonts.  Only