impstats does not document format options

[davidelang]

trafficstars

https://www.rsyslog.com/doc/v8-stable/configuration/modules/impstats.html

Format
type	default	mandatory	obsolete legacy directive
word	legacy	no	none
New in version 8.16.0.

Specifies the format of emitted stats messages. The default of “legacy” is compatible with pre v6-rsyslog. The other options provide support for structured formats (note the “cee” is actually “project lumberjack” logging).

The json-elasticsearch format supports the broken ElasticSearch JSON implementation. ES 2.0 no longer supports valid JSON and disallows dots inside names. The “json-elasticsearch” format option replaces those dots by the bang (“!”) character. So “discarded.full” becomes “discarded!full”. Options: json/json-elasticsearch/cee/legacy

This does not list json for example, and figuring out the other formats it supports is hard. There should be a simple list of the valid values for this field