Add note to 'Migrate to ILM' about existing indices (elastic#3509)

Just a tiny PR to help folks find our page about applying an ILM policy
to existing indices, and also reiterating the warning about not applying
a policy that includes rollover.

Closes: elastic#1567

<img width="589" height="302" alt="manage-existing"
src="https://github.com/user-attachments/assets/08e97370-35f2-474b-aadf-e772336ba8c6"
/>

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Co-authored-by: shainaraskas <[email protected]>

Author: 2 people

manage-data/lifecycle/index-lifecycle-management.md

@@ -101,3 +101,5 @@ For existing hot-warm deployments that are currently using index curation, migra
 * [Manage existing indices](/manage-data/lifecycle/index-lifecycle-management/manage-existing-indices.md)
 * [Migrate to index lifecycle management](/manage-data/lifecycle/index-lifecycle-management/migrate-index-management.md)
 * [Migrate index allocation filters to node roles](/manage-data/lifecycle/index-lifecycle-management/migrate-index-allocation-filters-to-node-roles.md)
+
+You can also set up {{ilm-init}} to manage an existing set of indices that do not already have a managed lifecycle. The {{ilm-init}} policy that you apply should not contain a rollover action, because the policy won't be carried forward when the rollover action creates a new index. Refer to [Manually apply a lifecycle policy to an index](/manage-data/lifecycle/index-lifecycle-management/policy-apply.md) to learn more.