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

lib/libc/gen/syslog.3

@@ -28,7 +28,7 @@
 .\"     @(#)syslog.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd November 25, 2018
+.Dd May 13, 2021
 .Dt SYSLOG 3
 .Os
 .Sh NAME
@@ -159,7 +159,7 @@ Log the process id with each message: useful for identifying
 instantiations of daemons.
 On
 .Fx ,
-this option is enabled by default.
+this option is enabled by default and cannot be disabled.
 .El
 .Pp
 The