docs: describe how to apply new ILM policies to existing backing indices

[axw]

https://www.elastic.co/guide/en/apm/guide/current/ilm-how-to.html#data-streams-custom-four describes how to create a custom ILM policy. The last step involves performing an optional rollover, which only applies the change to new indices. We should also describe how to change the ILM policy for existing backing indices.