Clarify Exposure SQL Model use cases

[pavlokhrebto]

What does this PR do? What is the motivation?

Motivation: Source product changes support using Exposure SQL Models to supply exposure records for Datadog Feature Flag experiments when decision metrics use warehouse data.

Source PRs:
https://github.com/ddoghq/dd-source/pull/49220
https://github.com/DataDog/web-ui/pull/340582

Changes:

• Updated hugo/content/en/experiments/concepts/exposure_sql.md to explain that Exposure SQL Models support bring-your-own randomization and Datadog Feature Flag experiments with warehouse decision metrics.
• Documented that the model must map the experiment subject type and every subject type required by the analysis so exposures can be joined to metric data.

Testing

• Ran git diff --check.
• Attempted vale hugo/content/en/experiments/concepts/exposure_sql.md, but vale is not installed in this sandbox.

Merge readiness

• Ready for merge

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AI assistance

Used Bits Code to draft and apply the documentation edit; manually reviewed the diff.

Additional notes

None.

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PR by Bits - View session in Datadog

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[datadog-official]

View session in Datadog

Bits Code status: ✅ Done

CI Auto-fix: Disabled | Enable

Comment @DataDog to request changes

[datadog-official]

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[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/dd/pavlo.khrebto/exposure-sql-flag-exposures/experiments/concepts/exposure_sql

[evazorro]

Thanks for updating the docs!