Document idle_in_transaction_session_timeout

[mriedem]

Simple documentation for the pass-through connection parameter added in https://github.com/brianc/node-postgres/pull/2049.

The description is taken from https://www.postgresql.org/docs/current/runtime-config-client.html.

Closes https://github.com/brianc/node-postgres/issues/2381

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[brianc]

nice! thanks!