[APM Integration/Server] Document to avoid touching the ILM policy of `traces-apm.sampled*` data stream

[lucabelluccini]

We should recommend to not change the ILM policy of the traces-apm.sampled* data stream as its indices should always be in the hot tier as much as possible.
This is a crucial "workspace" index for Tail Based Sampling and it has to be fast.

Ideally, we should avoid users making changes to the ILM policy of this specific data stream.
Still, as this index follows the Index Template structure similar to the one of traces-apm* there are several component templates in common. E.g. if someone edits traces@custom, the changes will be applied to all data streams traces-*, including the sampled one.

We could provide an hint of not touching the ILM policy of the sampled data stream in https://www.elastic.co/docs/solutions/observability/apm/index-lifecycle-management

KB associated to this https://support.elastic.co/knowledge/7364d176