Update release checklist so WordPress.tv doesn't publicize release video

[RMarks]

Per this conversation in Slack:
​https://wordpress.slack.com/archives/core/p1449616388004110

We need to add to the release checklist specifically to uncheck the Publicize options on wordpress.tv. I think the same thing happened with 4.2.

I can try to update this if pointed to where/how.

[samuelsidler]

Docs changes like these shouldn't be filed as meta tickets. If you have a change to docs, consult the relevant owner of that documentation (or, in the case of the Codex, edit it yourself).

That said, I updated the list in the core handbook.