`someUniqueSessionKey` in README/docs, but not in example

[davidmaxwaterman]

https://www.webcomponents.org/element/PolymerElements/app-storage

and

https://github.com/PolymerElements/app-storage/blob/master/README.md

have text:

<app-indexeddb-mirror
    session="a-unique-session-key-413"
    key="cats"
    data="{{liveData}}"
    persisted-data="{{persistedData}}">
</app-indexeddb-mirror>
When someUniqueSessionKey changes to a different value, app-indexeddb-mirror will delete the local data it is persisting at the "cats" key.

The text someUniqueSessionKey seems to refer to some property/attribute in the example, but that text doesn't appear in the example at all. I might assume it is a-unique-session-key-413, but that isn't clear at all.

Please consider using a binding for the session property too, eg:

  session="[[someUniqueSessionKey]]``

if that is indeed how it is intended.