Include text for the WG to allow it to go into maintenance and new feature mode after the end of the charter timeframe.

[afs]

Ensure the charter allows the WG to continue beyond the end of the scheduled timeframe in "maintenance and new feature" mode.
that allows new features (class 4 changes).

This includes errata.

https://www.w3.org/policies/process/#revised-rec-features

(RDF 1.2 is aiming to do this)