nixos/manual: Document PostgreSQL socket change
nixos/manual: Document PostgreSQL socket change

This is a backwards-incompatible change and while it won't probably
affect a whole lot of users, it makes sense to give them a heads-up
anyway.

Signed-off-by: aszlig <ee1aa092358634f9c53f01b5a783726c9e21b35a@nix.build>

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