docs: document peer authentication over unix sockets

[Hashim1999164]

Summary

• Expands the unix socket connection docs to explain that host should be the socket directory.
• Adds a peer authentication (password-less local) example using /var/run/postgresql, matching the request in Connection Documentation: Peer Conncetion #1797.
• Fixes the existing unix socket snippet so the client is declared with const.

Closes #1797

Test plan

• Preview the Connecting docs page and confirm the new Peer authentication section renders correctly
• Spot-check that the example matches local peer auth setups (host = socket dir, no password)

[charmander]

I don’t think the extra section adds much correct value. How about just:

Connections to unix sockets can also be made by setting host to the directory that contains the PostgreSQL socket (commonly /run/postgresql or /tmp). Unix sockets can support peer authentication without a password.

(there might be a better link, but it’s nothing node-postgres-specific)