syslog.3 - Add detail about LOG_PID

Document that LOG_PID is ignored and can not be disabled.
This change was made along with the move from RFC 3164 to RFC 5424 log messages.

PR: 255664
Reported by: des.gaufres@gmail.com
Reviewed by: gbe, jilles
Approved by: gbe (mentor, manpages), jilles