[Website]: Document / check what happens when not providing es_datastream_name

### Type of issue

None

### What documentation page is affected

https://www.elastic.co/docs/reference/integrations/awsfirehose

### What happened?

We should clarify what happens if someone doesn't define `es_datastream_name`.

From the [docs](https://www.elastic.co/docs/reference/integrations/awsfirehose#:~:text=By%20default%2C%20logs%20will%20be%20stored%20in%20logs%2Dawsfirehose%2Ddefault%20data%20stream%20and%20metrics%20will%20be%20stored%20in%20metrics%2Daws.cloudwatch%2Ddefault%20data%20stream.), it looks like it should go to `logs-awsfirehose-default` data stream, but from a recent test it seems it doesn't and requests are rejected with a 400 error.

Still, it's unclear what data stream will be used when shipping logs or metrics not supported by the integration.
- Or we're not honoring the default `logs-awsfirehose-default` data stream
- Or the setting `es_datastream_name` is mandatory when shipping logs or metrics not supported by AWS Firehose integration.

### Additional info

This requires engineering check prior to be documented.