Update docs for metadata_processors (#13650)

libbeat/docs/processors-using.asciidoc

@@ -1276,8 +1276,13 @@ section.
 === Add Kubernetes metadata
 
 The `add_kubernetes_metadata` processor annotates each event with relevant
-metadata based on which Kubernetes pod the event originated from. Each event is
-annotated with:
+metadata based on which Kubernetes pod the event originated from.
+At startup it detects an `in_cluster` environment and caches the
+Kubernetes-related metadata. Events are only annotated if a valid configuration
+is detected. If it's not able to detect a valid Kubernetes configuration,
+the events are not annotated with Kubernetes-related metadata.
+
+Each event is annotated with:
 
 * Pod Name
 * Pod UID
@@ -1360,7 +1365,11 @@ case you want to specify your own.
 === Add Docker metadata
 
 The `add_docker_metadata` processor annotates each event with relevant metadata
-from Docker containers:
+from Docker containers. At startup it detects a docker environment and caches the metadata.
+The events are annotated with Docker metadata, only if a valid configuration
+is detected and the processor is able to reach Docker API.
+
+Each event is annotated with:
 
 * Container ID
 * Name