eventpb: include field type in notable event types docs

The [Notable Event Types](https://www.cockroachlabs.com/docs/stable/eventlog) docs page lists all the fields for each event but doesn't say what type the field is. It would be useful for the field type to be documented so that a consumer of the events doesn't need to guess the type or determine it through trial and error.

Note: I'm not filing this as a docs issue because the docs are auto-generated by code.

Jira issue: CRDB-44702

Epic CRDB-42978