[DOCS] Warn APM Index Template cloning is dangerous #1851
Description
Cloning index templates as we suggest at https://www.elastic.co/guide/en/observability/8.13/apm-ilm-how-to.html#apm-data-streams-custom-policy-namespace is extremely risky as the user will be "stuck" in the past.
We already had several situations where any user who cloned index templates ended up in not picking up the newer index templates.
This led to those known issues in the past:
- 8.6 | Index Templates moved the index.default_pipeline from the main index template to the @Package index template https://support.elastic.co/knowledge/65e78ded
- 8.12 | @Custom ingest pipelines reorganization/rename might break APM and other integrations https://support.elastic.co/knowledge/594ba2c6
- 8.13 | Introduction of ecs@mappings in all Index Templates managed by Fleet is not picked up on cloned templates https://support.elastic.co/knowledge/df0eaa25
- 8.15 | Switch to Elasticsearch-managed Index Templates for APM is not picked up by users who cloned templates https://support.elastic.co/knowledge/a37653cf
We should document/warn that any customization done via cloned index templates would be to be "repeated" every upgrade.