Improve documentation

Author: rdner

api/shipper.pb.go

@@ -57,9 +57,13 @@ message Event {
 // Source information required for proper event tracking, processing and routing
 message Source {
  // Input ID in the agent policy.
+ // This value should be added to each produced event and forwarded via this proto field.
+ // When running under agent a processor is generated for adding this metadata.
  string input_id = 1;
  // Stream ID in the agent policy (Optional, some inputs don't use streams).
  // Not to be confused with data streams in Elasticsearch.
+ // This value should be added to each produced event and forwarded via this proto field.
+ // When running under agent a processor is generated for adding this metadata.
  string stream_id = 2;
 }