Update create-detection-rule.md

[bczifra]

Explicitly noted that hits aren't available in the threshold rule results context, indicated that context.alerts.kibana.alert.threshold_result.terms contains fields and values from any group by specified by the rule.

[nastasha-solomon]

Thanks for opening this @bczifra! @yctercero can someone from the DE team review these changes?

[github-actions]

🔍 Preview links for changed docs

• solutions/security/detect-and-alert/create-detection-rule.md

[florent-leborgne]

I'll merge this as it's been open for quite some time already. I think we found a good way to word this. We can improve more later if needed by raising another PR. Thanks!