Postfix: Add note about timestamps

[miwent]

Postfix events will include a timestamp that is commonly a legacy syslog format without a year. When parsed and indexed, Opensearch will treat the year as 1970.

In order to address this a processing step was added which reformats the date with the current year. See https://github.com/Graylog2/graylog-project-illuminate/issues/2039.

Add a note about this process, and that the potential exists for the year to be marked incorrectly for the seconds around the changing of the year, or when replaying older logs.