logger.1: Clarify -i flag

The PID of the logger process is always recorded along with the move from RFC
3164 to RFC 5424.

Indicate the -i flag is a no-op now in logger(1).

Related to a similary change in syslog.3

Reported by: danfe@
Approved by: manpages (0mp)
Fixes: 94eb6c42ab4a
Differential Revision: https://reviews.freebsd.org/D34578