improve doc for fromStateSetting

[pixelzoom]

The documentation for fromStateSetting doesn't tell me anything useful. I have no idea what it does, or how it's related to validation.

In PhetioGroup.js and PhetioDynamicElementContainer.js:

   * @param {boolean} [fromStateSetting] - used for validation during state setting.
  ...
  disposeElement( element, fromStateSetting ) {

In PhetioGroup.js:

   * @param {boolean} [fromStateSetting] - used for validation during state setting.
   * @returns {PhetioObject}
   * @public (PhetioGroupIO)
   */
  createIndexedElement( index, argsForCreateFunction, fromStateSetting ) {

In PhetioGroup.js, as an option to method clear:

    options = merge( {
      fromStateSetting: false, // used for validation during state setting (phet-io internal)

[samreid]

I took a look around but am not clear what is happening here. @zepumph can you please take a look?

[zepumph]

Yes, this is a confusing parameter, and there isn't a clear single place to document all usages of it. I did the best I could of pointing to a single explanation. @pixelzoom please review.

[pixelzoom]

Much better documentation, thanks. I fixed a few typos in the above commit. Closing.