Adjustments to the field set "usage" docs

[djptek]

• Have you signed the contributor license agreement? Y
• Have you followed the contributor guidelines? Y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? Y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? Y
• Is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed. Y
• Have you added an entry to the [CHANGELOG.next.md] (https://github.com/elastic/ecs/blob/master/CHANGELOG.next.md)? WIP

See #1072

[ebeahan]

One comment about the changelog, otherwise LGTM

[djptek]

1 line change to CHANGELOG.next.md applied, all tests passed, merging