syslog.3: MFC: 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.

(cherry picked from commit 5aee3e14d4914c7c99bce80da17b3100cb1f4490)

2 files changed, 14 insertions, 2 deletions

diff --git a/lib/libc/gen/syslog.3 b/lib/libc/gen/syslog.3
index 2cb2c0d198b3..73e62997e828 100644
--- a/lib/libc/gen/syslog.3
+++ b/lib/libc/gen/syslog.3
@@ -28,7 +28,7 @@
 .\"     @(#)syslog.3	8.1 (Berkeley) 6/4/93
 .\" $FreeBSD$
 .\"
-.Dd November 25, 2018
+.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
diff --git a/usr.bin/logger/logger.1 b/usr.bin/logger/logger.1
index 7a64320cfe49..a2774b9e3500 100644
--- a/usr.bin/logger/logger.1
+++ b/usr.bin/logger/logger.1
@@ -28,7 +28,7 @@
 .\"	@(#)logger.1	8.1 (Berkeley) 6/6/93
 .\" $FreeBSD$
 .\"
-.Dd December 5, 2017
+.Dd July 3, 2023
 .Dt LOGGER 1
 .Os
 .Sh NAME
@@ -73,6 +73,9 @@ tries to send the message to all addresses.
 .It Fl i
 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
@@ -152,6 +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