Update otelcol.exporter.kafka.md

[lomobot]

PR Description

Changes were added in otelcol.exporter.kafka.md

Notes to the Reviewer

Current doc markup structure for otelcol.exporter.kafka module doesn't describes that plaintext, sasl, aws_msk, tls, kerberos must be included in authentication block and retry block in metadata. I'd try to highlight this fact by using markup for this blocks.

PR Checklist

• Documentation added

[clayton-cornell]

Thanks for this suggestion. :-)

This change isn't consistent with the other component documentation topics (there are a few topics that do nest the blocks as a child heading in the doc structure, but not all do this). The block nesting is shown in the Blocks table here: https://grafana.com/docs/alloy/latest/reference/components/otelcol/otelcol.exporter.kafka/#blocks

I agree the presentation in the individual block descriptions doesn't make it clear what has to go where. I've got an open task right now to revisit how we document this exact part of the component documentation. So, I'd like to put this PR on hold for a couple of days until I've had some time to revisit this suggestion - if this is the way forward, then we can either add other topics with similar issues to this PR or open up a new PR to capture the change in the other topics (keeps things consistent)