swscale/swscale: Document param[0..1]

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/swscale.h

@@ -185,6 +185,12 @@ void sws_freeContext(struct SwsContext *swsContext);
  * @param dstH the height of the destination image
  * @param dstFormat the destination image format
  * @param flags specify which algorithm and options to use for rescaling
+ * @param param extra parameters to tune the used scaler
+ *              For SWS_BICUBIC param[0] and [1] tune the shape of the basis
+ *              function, param[0] tunes f(1) and param[1] f´(1)
+ *              For SWS_GAUSS param[0] tunes the exponent and thus cutoff
+ *              frequency
+ *              For SWS_LANCZOS param[0] tunes the width of the window function
  * @return a pointer to an allocated context, or NULL in case of error
  * @note this function is to be removed after a saner alternative is
  *       written