adds warning against Postgres across NFS

What does this MR do?

Adds a stern warning against using Postgres across NFS.

What are the relevant issue numbers?

None, unless you'd like a ZenDesk link for customers who had this configuration :)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides