[DOCS] Adds missing_bucket setting to transform APIs

[szabosteve]

Overview

Related to: #42941

This PR adds the missing_bucket setting to the transform group_by property. It also amends the example API call for PUT transform to demonstrate how to use missing_bucket.

Preview

PUT transform

[github-actions]

Documentation preview:

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[elasticsearchmachine]

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

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[hendrikmuhs]

With this change missing_bucket is listed as Properties of group_by, but that is one level higher, missing_bucket is a property of terms, date_histogram, histogram or geotile_grid. We are just linking those and missing_bucket is documented there. This is a problem, I fully agree that we should promote missing_bucket or at least document that per default groupings without a value are ignored. Can we do this in the description of group_by?

[szabosteve]

@hendrikmuhs Thanks for the clarification. In the meantime, I also realized that the property was a level "higher" than it should. I was thinking a bit about where to document it; to add it to the group by description or to add a note somewhere else. Finally, I've added two extra sentences to the group_by description I think that's the most obvious place to put it. Let me know if you think we should modify anything.
(Addressed in 1270bee.)

[hendrikmuhs]

LGTM! Thanks @szabosteve

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
✅	8.1
✅	8.2
✅	8.3
❌	7.17	Commit could not be cherrypicked due to conflicts
✅	8.4

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 90111