nixos/prometheus-dovecot-exporter: enhance `socketPath` documentation

In Dovecot 2.3[1] the stats module changed and now the UNIX socket
provided by Dovecot by default isn't compatible anymore with the
exporter[2]. By enabling the `old-stats` plugin in Dovecot this issue
can be solved which should be documented in this module.

[1] https://wiki2.dovecot.org/Upgrading/2.3
[2] https://github.com/kumina/dovecot_exporter/issues/8