Fix #4352, document pref required to install dev Screenshots on some profiles
Fix #4352, document pref required to install dev Screenshots on some profiles

Also removes reference to extensions.screenshots.system-disabled which we no longer use

mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/pageshot