db_postgres document how to use it with unix socket (#15187)

lib/impure/db_postgres.nim

@@ -37,6 +37,26 @@
 ##                 VALUES ($1, $2, $3)""",
 ##           3)
 ##
+##
+## Unix Socket
+## ===========
+##
+## Using Unix sockets instead of TCP connection can
+## `improve performance up to 30% ~ 175% for some operations <https://momjian.us/main/blogs/pgblog/2012.html#June_6_2012>`_.
+##
+## To use Unix sockets with `db_postgres`, change the server address to the socket file path:
+##
+## .. code-block:: Nim
+##   import db_postgres ## Change "localhost" or "127.0.0.1" to the socket file path
+##   let db = db_postgres.open("/run/postgresql", "user", "password", "database")
+##   echo db.getAllRows(sql"SELECT version();")
+##   db.close()
+##
+## The socket file path is operating system specific and distribution specific,
+## additional configuration may or may not be needed on your `postgresql.conf`.
+## The Postgres server must be on the same computer and only works for Unix-like operating systems.
+##
+##
 ## Examples
 ## ========
 ##