Change docs to be consistent with script

Summary:
Ref https://discourse.phabricator-community.org/t/upgrade-article-process-doesnt-match-script/2598.

I personally prefer "stop the server and then pull" to reduce the risk of scary "served half the request from one version and half from another", but I think this is hard to pull off with correctly configured APC caches.

Test Plan: Mk.1 eyeball

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20391