Document reason for datastream timestamp field name

[cthtrifork]

Is your feature request related to a problem? Please describe.
I have tried looking at the code and the documentation for datastream. I can not find documentation that describes why a timestamp field is needed on each document.
Rollover is based on index timestamp and each new index is created with a generationId. I can not spot anywhere the timestamp on a document has a technical impact.

Describe the solution you'd like
Update https://opensearch.org/docs/2.10/im-plugin/data-streams/ with a technical reason for the timestamp requiremement on documents.

Describe alternatives you've considered
Make the field optional if it has no functional impact

[noCharger]

@opensearch-project/admin Please transfer this to https://github.com/opensearch-project/documentation-website. Thanks!

[Naarcha-AWS]

@noCharger: Is there a technical reason for need on timestamp on each document?

[cthtrifork]

@noCharger I am permitting myself to bump this, hope it is okay,

[cthtrifork]

This seems related: #2811

[hdhalter]

HI @bowenlan-amzn, can you please help us update the documentation for this issue?

[bowenlan-amzn]

Data stream function is for time-series data, so requiring the timestamp field. It's not a functional issue, but just the feature.
@ketanv3 would the data stream expert to help explain more.