Add a note under Proposals -> Process -> "Gather feedback [...]" about getting "next steps" in #matrix-spec

A small discussion with @turt2live yielded that, to continue work on a proposal after points have been addressed (but there seems to be no activity on the MSC from SCT members), the courtesy is to ask in `#matrix-spec:matrix.org` about it's next steps and current (unresolved) issues.

I think adding the following passage under Proposals -> Process -> "Gather feedback as widely as possible." could help this, as it's supposed to be part of the official process;
> "When getting stuck or finding no points in the discussion to answer, ask patiently in #matrix-spec:matrix.org for guiding points on how to progress the proposal forward. Reminder; please be patient."

Locate Proposals → Process → “Gather feedback as widely as possible.” in the specification and review the surrounding process guidance. Add the proposed note about asking in #matrix-spec:matrix.org for next steps; the work is done when that guidance appears under the specified section.

Written by the indexing model from the issue text.