freetype.h (FT_Set_Transform): Minor documentation update.

In order to mention the limited accuracy of `FT_Matrix' coefficients
and encourage client code to perform the transform themselves with
floating point values instead.

include/freetype/freetype.h

@@ -3175,6 +3175,12 @@ FT_BEGIN_HEADER
    *     A pointer to the translation vector.  Use `NULL` for the null vector.
    *
    * @note:
+   *   This function is provided as a convenience, but keep in mind that
+   *   @FT_Matrix coefficients are only 16.16 fixed point values, which can
+   *   limit the accuracy of the results.  Using floating-point computations
+   *   to perform the transform directly in client code instead will always
+   *   yield better numbers.
+   *
    *   The transformation is only applied to scalable image formats after the
    *   glyph has been loaded.  It means that hinting is unaltered by the
    *   transformation and is performed on the character size given in the