[Bug] `ariaNotify` is not implemented on `Document`

[smockle]

Problem

The ariaNotify explainer includes several examples of calling document.ariaNotify, so this should be supported by the polyfill. However, the polyfill only implements ariaNotify on Element (document instanceof Element is false).

Tasks

• Duplicate lines 169–179, but for Document.prototype.

Workaround

Until this issue is resolved, use document.documentElement.ariaNotify or document.body.ariaNotify instead of document.ariaNotify.

[smockle]

Opened #22 to fix this!