loki.source.docker: document positions file usage (grafana#4686)

Co-authored-by: Clayton Cornell <131809008+clayton-cornell@users.noreply.github.com>

docs/sources/flow/reference/components/loki.source.docker.md

@@ -64,6 +64,12 @@ configuration.
 * `loki_source_docker_target_entries_total` (gauge): Total number of successful entries sent to the Docker target.
 * `loki_source_docker_target_parsing_errors_total` (gauge): Total number of parsing errors while receiving Docker messages.
 
+## Component behavior
+The component uses its data path (a directory named after the domain's
+fully qualified name) to store its _positions file_. The positions file
+stores the read offsets so that if there is a component or Agent restart,
+`loki.source.docker` can pick up tailing from the same spot.
+
 ## Example
 
 This example collects log entries from the files specified in the `targets`