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)