11093 fix

See https://trac.openmicroscopy.org.uk/ome/ticket/11093 - warning about no recovery without database added to server backup and restore docs.

(Added to both sections incase someone skips straight to the PostgreSQL backup section in the page contents menu and misses the first warning box)

I can add to the troubleshooting page as well if anyone thinks that is necessary, I thought this would probably do but I can also add an actual FAQ to the website if needed.