📚 Adding documentation for configuration

This builds on a [question asked in Slack][1]

[1]: https://samvera.slack.com/archives/C03S9FS60KW/p1705681632335919

lib/bulkrax.rb

@@ -19,7 +19,6 @@ class Configuration
                   :export_path,
                   :field_mappings,
                   :file_model_class,
-                  :fill_in_blank_source_identifiers,
                   :generated_metadata_mapping,
                   :import_path,
                   :multi_value_element_join_on,
@@ -35,6 +34,11 @@ class Configuration
                   :reserved_properties,
                   :server_name
 
+    ##
+    # @return [#call] with arity 2.  The first parameter is a {Bulkrax::ApplicationParser} and the
+    #         second parameter is an Integer for the index of the record encountered in the import.
+    attr_accessor :fill_in_blank_source_identifiers
+
     attr_writer :use_locking
 
     def use_locking