Improve docs for correlation key for message start event

[mesellings]

Description

This PR relates to and closes issue https://github.com/camunda/developer-experience/issues/378.

It relates to the bug raised: camunda/camunda-modeler#4466.

• Added a note and some text edits to help clarify the documentation, as customer feedback indicates a misunderstanding is occurring about the correlation key.

Preview environment
https://preview.docs.camunda.cloud/pr-4336/docs/next/components/modeler/bpmn/message-events/

Note: There may be further work involved in this area of the docs (see camunda/camunda#2718) but this PR gets the clarifying note out as a first iteration.

When should this change go live?

• This is a bug fix, security concern, or something that needs urgent release support.
• This is already available but undocumented and should be released within a week.
• This on a specific schedule and the assignee will coordinate a release with the DevEx team. (apply hold label or convert to draft PR)
• This is part of a scheduled alpha or minor. (apply alpha or minor label)
• There is no urgency with this change and can be released at any time.

PR Checklist

• My changes are for an already released minor and are in /versioned_docs directory.
• My changes are for the next minor and are in /docs directory (aka /next/).

• My changes require an Engineering review, and I've assigned the Engineering DRI or delegate.
• My changes require a technical writer review, and I've assigned @camunda/tech-writers as a reviewer.

[github-actions]

👋 🤖 ✅ Looks like the changes were ported across versions, nice job! 🎉

You can read more about the versioning within our docs in our documentation guidelines.

[github-actions]

The preview environment relating to the commit 66823a0 has successfully been deployed. You can access it at https://preview.docs.camunda.cloud/pr-4336/index.html