Update Status of This Document section

[anssiko]

Update Status of This Document section to better communicate the stability of the specification.

The current text boilerplate in bold is a leftover from the First Public Working Draft-era (Oct 2014). Since the FPWD, multiple updated Working Drafts have been published, the spec has received experimental implementation feedback and we believe is getting close to feature completeness. We should expect little spec churn now due to a tighter scope we've settled on.