Add documentation for CloudFoundry application metadata persistent ca…

…che (#22669)

* Add documentation for CF application metadata persistent cache

* Add explanation for metadata persistent cache

x-pack/libbeat/processors/add_cloudfoundry_metadata/docs/add_cloudfoundry_metadata.asciidoc

@@ -25,6 +25,10 @@ metadata in all events from the firehose since version 2.8. In these cases the
 metadata in the events is used, and `add_cloudfoundry_metadata` processor
 doesn't modify these fields.
 
+For efficient annotation, application metadata retrieved by the Cloud Foundry
+client is stored in a persistent cache on the filesystem under the `path.data`
+directory. This is done so the metadata can persist across restarts of {beatname_uc}.
+For control over this cache, use the `cache_duration` and `cache_retry_delay` settings.
 
 [source,yaml]
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