Document the option to use painless as script language in search templates

[laraMorenoIgle]

Painless can be used as script language for search templates as done in this github #24529.
However as per our documentation mustache is the only allowed language for creating search templates.
If painless is fully supported as script language in search templates, then we should state it in the documenation and provide at least one example.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Scripting)

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[jdconrad]

If we do want to officially support this, we need to add some test coverage to ensure changes to Painless don't break templates unknowingly. We may also want to consider the places that watcher hard codes mustache for template support.

[jdconrad]

Spoke with @stu-elastic and @rjernst - we've agreed this should be supported, so it's a matter of tests and docs now.

[sauln]

Hey, I'm stoked to get to use Painless in my search templates. Is there any draft documentation on how this works? I'm not sure if the syntax described in #24529 is relevant at all.

Thank you,

[stu-elastic]

Painless cannot be used in search templates and we do not currently have plans to add the functionality necessary to make it useful in that context.

If there are shortcomings in mustache that need to be addressed, we'd love to see them, with use cases, in separate issues.

Since the documentation on the search template API is correct,

Allows you to use the mustache language to pre render search requests.

I'm going to close this issue.