[sfnt] Correct FT_ColorStopIterator documentation.

* include/freetype/ftcolor.h (FT_ColorStopIterator): the user should not
 set `p` to NULL before calling `FT_Get_Colorline_Stops`. `p` and
`num_color_stops` are set by `FT_Get_Paint`. `p` and
`current_color_stop` are updated by `FT_Get_Colorline_Stops`.

include/freetype/ftcolor.h

@@ -528,14 +528,14 @@ FT_BEGIN_HEADER
    * @fields:
    *   num_color_stops ::
    *     The number of color stops for the requested glyph index.  Set by
-   *     @FT_Get_Colorline_Stops.
+   *     @FT_Get_Paint.
    *
    *   current_color_stop ::
    *     The current color stop.  Set by @FT_Get_Colorline_Stops.
    *
    *   p ::
-   *     An opaque pointer into 'COLR' table data.  The caller must set this
-   *     to `NULL` before the first call of @FT_Get_Colorline_Stops.
+   *     An opaque pointer into 'COLR' table data.  Set by @FT_Get_Paint.
+   *     Updated by @FT_Get_Colorline_Stops.
    *
    * @since:
    *   2.11 -- **currently experimental only!**  There might be changes