docs(external): add note for input/output typs in remap docs

[pront]

Summary

Rephrases and clarifies on event types.

P.S. I also noticed that inputs is not rendered.

Change Type

• Bug fix
• New feature
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

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Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the "no-changelog" label to this PR.

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References

Ref: #19508 (comment)

[rtrieu]

One nit, but otherwise looks good.

[bfung]

editorial suggestions for the remap body text.

[datadog-vectordotdev]

Datadog Report

Branch report: pront/remap-docs-improv
Commit report: c854048
Test service: vector

✅ 0 Failed, 7 Passed, 0 Skipped, 25.52s Total Time