Add note about syncdb/migrate idempotency to upgrading doc.
Add note about syncdb/migrate idempotency to upgrading doc.

bobsilverberg/moztrap,mozilla/moztrap,shinglyu/moztrap,mozilla/moztrap,mozilla/moztrap,bobsilverberg/moztrap,mozilla/moztrap,mccarrmb/moztrap,bobsilverberg/moztrap,mccarrmb/moztrap,shinglyu/moztrap,shinglyu/moztrap,mccarrmb/moztrap,bobsilverberg/moztrap,mccarrmb/moztrap,mccarrmb/moztrap,shinglyu/moztrap,shinglyu/moztrap,mozilla/moztrap