Add reference to prefer-ilm setting for apm lifeycle known bug

docs/en/observability/apm/known-issues.asciidoc

@@ -91,7 +91,10 @@ used. One of the following approaches can be adopted to fix the unmanaged indice
 . Manually delete the indices when they are no longer needed.
 . Explicitly configure APM data streams with the default data stream lifecycle config.
 Using this approach would migrate all data streams to use data stream lifecycles,
-which should be equivalent to the default ILM policies:
+which should be equivalent to the default ILM policies. Note that if you have a custom
+lifecycle policy in place for any of the datastreams then you don't need to do anything
+for those datastreams as they are not affected by this bug. Going forward, for using
+custom ILM policies with new datastreams, please consult the {ref}/apm-ilm-how-to.html[updated guide].
 +
 [source,txt]
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