Adds note on monic filter

silk/fixed/noise_shape_analysis_FIX.c

@@ -34,6 +34,9 @@ POSSIBILITY OF SUCH DAMAGE.
 
 /* Compute gain to make warped filter coefficients have a zero mean log frequency response on a     */
 /* non-warped frequency scale. (So that it can be implemented with a minimum-phase monic filter.)   */
+/* Note: A monic filter is one with the first coefficient equal to 1.0. In Silk
+   we omit the first coefficient in an array of coefficients, for monic filters.
+*/
 static inline opus_int32 warped_gain( /* gain in Q16*/
     const opus_int32     *coefs_Q24,
     opus_int             lambda_Q16,

silk/float/noise_shape_analysis_FLP.c

@@ -34,6 +34,9 @@ POSSIBILITY OF SUCH DAMAGE.
 
 /* Compute gain to make warped filter coefficients have a zero mean log frequency response on a     */
 /* non-warped frequency scale. (So that it can be implemented with a minimum-phase monic filter.)   */
+/* Note: A monic filter is one with the first coefficient equal to 1.0. In Silk
+   we omit the first coefficient in an array of coefficients, for monic filters.
+*/
 static inline silk_float warped_gain(
     const silk_float     *coefs,
     silk_float           lambda,