Fix inventory threshold rule preview description in serverless

[dedemorton]

Ports #3941 to serverless.

Preview is here: https://docs-elastic-e47kjs4tp-elastic-dev.vercel.app/serverless/observability/create-inventory-threshold-alert-rule

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @dedemorton? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-/d./d is the label to automatically backport to the /d./d branch. /d is the digit
  NOTE: backport-skip has been added to this pull request.

[elasticdocs]

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[mdbirnstiehl]

🐦