[8.13] Adds important callout about the possibility of multiple ELSER deployment (backport #2744)

[mergify]

Overview

It is possible to deploy ELSER multiple times by using different deployment IDs. The recommended behavior is to have different deployments with different purposes. This PR adds an important callout to the ELSER page to highlight these pieces of information.

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This is an automatic backport of pull request #2744 done by Mergify.

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