Add docs on replicating APM Server or Beats indices (#36333)

This commit adds a brief note to the documentation on how to manage the
index templates that are used to create APM Server and Beats indices.

docs/reference/ccr/requirements.asciidoc

@@ -42,4 +42,26 @@ The number of soft deletes to retain. Soft deletes are collected during merges
 on the underlying Lucene index yet retained up to the number of operations
 configured by this setting. The default value is `0`.
 
-For more information about index settings, see {ref}/index-modules.html[Index modules].
+For more information about index settings, see {ref}/index-modules.html[Index modules].
+
+[float]
+[[ccr-overview-beats]]
+==== Setting soft deletes on indices created by APM Server or Beats
+
+If you want to replicate indices created by APM Server or Beats, and are
+allowing APM Server or Beats to manage index templates, you need to enable
+soft deletes on the underlying index templates. To enable soft deletes on the
+underlying index templates, add the following changes to the relevant APM Server
+or Beats configuration file.
+
+["source","yaml"]
+----------------------------------------------------------------------
+setup.template.overwrite: true
+setup.template.settings:
+  index.soft_deletes.enabled: true
+  index.soft_deletes.retention.operations: 1024
+----------------------------------------------------------------------
+
+For additional information on controlling the index templates managed by APM
+Server or Beats, see the relevant documentation on loading the Elasticsearch
+index template.