doc(lib): add documentation for the dart side

lib/src/utils/exception.dart

@@ -2,17 +2,28 @@ import 'dart:io';
 
 import 'package:nearby_service/src/utils/logger.dart';
 
+///
+/// Nearby Service Plugin Exception.
+///
+/// Indicates what problem occurred in the plugin operation.
+///
 class NearbyServiceException implements Exception {
   NearbyServiceException(this.error) {
     Logger.error(error);
   }
 
+  ///
+  /// A call from an unsupported platform.
+  ///
   factory NearbyServiceException.unsupportedPlatform({required String caller}) {
     return NearbyServiceException(
       '$caller is not supported for platform ${Platform.operatingSystem}',
     );
   }
 
+  ///
+  /// A decoding error.
+  ///
   factory NearbyServiceException.unsupportedDecoding(dynamic value) {
     return NearbyServiceException(
       'Got unknown value=$value with runtimeType=${value.runtimeType}',