[Website]: Update Serverless differences

[inqueue]

Type of issue

Missing information

What documentation page is affected

https://www.elastic.co/docs/deploy-manage/deploy/elastic-cloud/differences-from-other-elasticsearch-offerings

What happened?

Some Serverless differences are undocumented and should be.

1. 5s is the lowest refresh interval configurable by an end user for an index.
2. 200ms is the minimum response time for a single bulk indexing request.

Additional info

No response

[shainaraskas]

need to verify with serverless pm

[shubhaat]

• @dbrimley , @giladgal