Remove inaccurate references to corruption, remove SEGTERM suggestion… (#1303)

* Remove inaccurate references to corruption, remove SEGTERM suggestion, update information links to current docs. Postgres is carefully designed such that data is not corrupted on crashes or unclean shutdowns - the main tradeoff is that WAL replay is needed on startup. In practice, SIGTERM can cause unexpected long delays to shutdowns - often during maintenance windows - so best not to actively suggest this. The links back to official Postgres documentation seem sufficient.