Document that elasticsearch output does not sprintf the datastream options

[graphaelli]

Description: There was another recent case of confusion similar to:

• https://discuss.elastic.co/t/use-field-name-in-datastream-namespace-or-dataset/302188/3
• https://discuss.elastic.co/t/dynamic-naming-of-elasticsearch-data-streams/325278/4
• Use metadata for data_stream_auto_routing #13528 (comment)

Let's clarify that this won't work from the docs.