docs: Clarify Elasticsearch output sample config

[bmorelli25]

The sample config file in the Elasticsearch output shows an index naming pattern ending in -%{+yyyy.MM.dd}. Multiple users have pointed out on discuss that this is confusing as ILM, which is enabled by default, uses a different index naming pattern.

We should add a code callout that clarifies this. This file should also be double-checked for other improvements as it hasn't been updated in a while.