Rework "Introduced in" event release history column

[magnusbaeck]

Description

The documentation of the "Introduced in" event release history column introduced in #325,

The first edition in which this version was the latest for this event type. If the version hasn't been released in a protocol edition or if there are newer versions in the first edition where it was introduced, use "No edition set".

is hard to understand, but that's arguably because the meaning of this column is weird and doesn't match the column heading. Either the column should be renamed to something that more closely matches its intended purpose, or "No edition set" should be changed to something better, or both.

Motivation

The current way this column is used is confusing to everyone.

Exemplification

N/A

Benefits

Clearer documentation, less confusion for those updating the table.

Possible Drawbacks

None.

[magnusbaeck]

How about we change the introduced_in key to contain either a tag name (e.g. "edition-arica") or be left out (effectively null), and update the documentation renderer accordingly? That's friendlier to machine processors, reduces the amount of Markdown in the event definition YAML files, and is a pretty simple change (including for the follow-up issue #327).

[e-backmark-ericsson]

I guess that would be fine