Warn that cloned templates are not automatically updated

[karenzone]

Closes: #1851

PREVIEW LINK: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/2740/solutions/observability/apm/index-lifecycle-management#apm-data-streams-custom-policy-namespace

[karenzone]

@lucabelluccini, we can definitely add a note to the docs.
Is there a compelling use case for cloning templates? This seems risky, and asking users to remember to repeat this process on every upgrade seems like a lot.

Is there a product enhancement request to make upgrading cloned templates easier? Or at least to check and warn?

[github-actions]

🔍 Preview links for changed docs

• solutions/observability/apm/index-lifecycle-management.md

[mdbirnstiehl]

LGTM!

[lucabelluccini]

I've left some suggestions.
I would like to "reinforce" the message that cloning index template is really necessary only when a user wants a customization per namespace.

The other info we might share here is that this @custom component template technique can be used also for other customizations (not just ILM).

I would also warn users explicitly to never edit the built-in/provided index templates nor to add extra component templates on top of the existing ones, or change the order of the component templates as it might break the correct behavior of the Observability applications relying on the APM data.