Address accessibility issues

[npm1]

Based on #91 (comment). Assistive devices could trigger synthetic input events while skipping the hardware input events that this API cares about. This means that we may not be exposing performance data for users of those devices. The tricky part is exposing meaningful information (for instance, is the Event timeStamp still representative of the time in which the user interacted with the page, or would some other timestamp need to be used instead)? We should dig some data about how often this occurs, what kinds of events are triggered this way, and based on that update the spec.

In the interim, we could add an a11y section and link to this issue.