Add just a tiny bit more detail about unix socket connection strings

[atrauzzi]

After reading this page: https://www.npgsql.org/doc/connection-string-parameters.html?q=Unix-domain%20sockets

It would be nice to have the following:

• A full example of a unix socket connection string
• Any details around whether the npgsql driver generates or appends /.s.PGSQL.5432 or if it needs to be specified explicitly
• An explanation as to why the port is still required when configuring socket connections

[roji]

This works similiar to how the PostgreSQL libpq host works (see docs): you specify the directory where the Unix-domain socket is, and Npgsql appends the file name, integrating the port number. That seems pretty clear from the doc text, no?