Cleans up the constructor for PeriodicWave based on the algorithm
given in
WebAudio#2147 (comment).

A few other minor cleanups in the algorithm description were also
done:
*  Move declaration of `[[real]]`, `[[imag]]`, and `[[normalize]]` to
   the beginning of the algorithm.
* Remove extra text for the members of the `PeriodicWaveOptions` to
  the algorithm so that everthing is all in one place.