services: dovecot: Fix unix_listeners and fifo_listeners path types.

* gnu/services/mail.scm (unix-listener-configuration)[path]
(fifo-listener-configuration)[path]: Change type from 'file-name' to 'string'.
* doc/guix.texi (Mail Services): Document it.

Signed-off-by: Clément Lassieur <clement@lassieur.org>