syslog.3: document ident[N] format

When libc switched to generation of logs as per RFC 5424,
that change broke application ability to insert specific process id
using ident[N] format, the feature existed for decades.
Some processes rely on it (including logger and syslogd).

Later the regression was fixed but the feature remained undocumented.
This change documents it.

MFC after:	1 week

lib/libc/gen/syslog.3

@@ -28,7 +28,7 @@
 .\"     @(#)syslog.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd May 13, 2021
+.Dd July 3, 2023
 .Dt SYSLOG 3
 .Os
 .Sh NAME
@@ -131,6 +131,11 @@ The
 .Fa ident
 argument
 is a string that will be prepended to every message.
+It may be formatted as
+.Fa ident[N]
+in which case decimal number
+.Fa N
+replaces the process id within messages.
 The
 .Fa logopt
 argument

usr.bin/logger/logger.1

@@ -28,7 +28,7 @@
 .\"	@(#)logger.1	8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd March 16, 2022
+.Dd July 3, 2023
 .Dt LOGGER 1
 .Os
 .Sh NAME
@@ -74,6 +74,8 @@ tries to send the message to all addresses.
 Log the process id of the logger process
 with each line.
 This flag is ignored and the process id is always logged.
+See also
+.Fl t .
 .It Fl s
 Log the message to standard error, as well as the system log.
 .It Fl f Ar file
@@ -153,6 +155,10 @@ and
 Mark every line in the log with the specified
 .Ar tag
 rather than the default of current login name.
+Use
+.Fl t Ar tag[N]
+to insert specific decimal process id instead of id of
+.Nm .
 .It Ar message
 Write the message to log; if not specified, and the
 .Fl f