[Transform] Clarify best practices around using timestamps in continuous transform

[przemekwitek]

Description

One user expressed confusion wrt which timestamp field should be used as date_histogram.field and sync.time.field.

The problem is that:

• on one hand we recommend the sync.time.field to be ingestion timestamp set by the ingest pipeline but
• on the other hand, we recommend setting date_histogram timestamp field and sync.time.field to the same value in order to take advantage of the align_checkpoints optimization.

Currently, there is no way of having both at the same time which leads to confusion. We should either fix it in code or at least set up expectations with users and give them guidance on how to best set these fields.

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)