nixos/doc: document how to allow-list tablespaces

It was brought up that the restricted file-system access breaks
tablespaces[1]. I'd argue that this is the desired behavior, the whole
point of the hardening is the lock the service down and I don't consider
tablespaces common enough to elevate privileges again. Especially since
the workaround is trivial as shown in the diff.

For completeness sake, this adds the necessary `ReadWritePaths` change
to the postgresql section of the manual.

This also adds a small correction about the state of
`ensurePermissions`.

[1] https://github.com/NixOS/nixpkgs/pull/344925#issuecomment-2521188907

0 files changed, 0 insertions, 0 deletions