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