Documentation should mention non-bundled postgresql should be the same version as the bundled one

The caveat being that upgrades will fail to backup the database, because pg_dump will refuse dumping a different version. The work-around is softlinking or replacing the bundled pg_dump* utilities to same-version ones of the external postgresql server. (which needs to be redone after every upgrade).

This becomes even a larger hurdle when running dockerized. So the docs should make a mention of this.