message: Add documentation for Tone.Envelope's decay and release minimum values.
  Fixes
diff: @@ -14,10 +14,11 @@ define(["../core/Tone", "../signal/Signal",
  * @param {Time} [attack] The amount of time it takes for the envelope to go from
  * 0 to it's maximum value.
  * @param {Time} [decay] The period of time after the attack that it takes for the envelope
  - * to fall to the sustain value.
  + * to fall to the sustain value. Value must be greater than 0.
  * @param {NormalRange} [sustain] The percent of the maximum value that the envelope rests at until
  * the release is triggered.
  * @param {Time} [release] The amount of time after the release is triggered it takes to reach 0.
  + * Value must be greater than 0.
  * @example
  * //an amplitude envelope
  * var gainNode = Tone.context.createGain();
  