[chore] Add clarification on event attributes vs body fields

[JamieDanielson]

Changes

• Add clarification describing when to use attributes and when to use body fields for an event
  • This was discussed briefly during SemConv SIG meeting, with the goal of helping clarify the structure of events.
• Rename data to body
  • The name of the body had gone through several iterations before deciding on the final term body. I'm not sure whether the word payload is still necessary here or if it's a relic of the previous term, so I've left it but could see renaming these to just body from payload (data). This can also be a separate PR if further discussion is warranted.

Merge requirement checklist

I cannot tell if this would require a changelog, happy to add if deemed appropriate.

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[JamieDanielson]

@open-telemetry/semconv-event-approvers please take a look!