document why field names under event_data may be unnamed (named param1, param2, etc)

[dedemorton]

For Windows platforms prior to Windows Vista, the parameters in event log messages were unnamed
Therefore, for those platforms, the output fields that appear under event_data are named param1, param2, param3, param4, etc.

See https://discuss.elastic.co/t/windows-2003-winlogbeat-x32-alpha4/57386/2

Clarification about this needs to be added here: https://www.elastic.co/guide/en/beats/winlogbeat/master/exported-fields-eventlog.html#_event_data

And perhaps an FAQ topic should also be added.

[dedemorton]

Closing in favor of tracking this issue in #2482