[PE] document `registerFixForRule`

Change-Id: Ie0f2f9bd85207ec6a25d881fbd8182c6506620f0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/408884
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>
Auto-Submit: Phil Quitslund <pquitslund@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analysis_server_plugin/lib/registry.dart

diff --git a/pkg/analysis_server_plugin/lib/registry.dart b/pkg/analysis_server_plugin/lib/registry.dart
index c656221..3fe23a9 100644
--- a/pkg/analysis_server_plugin/lib/registry.dart
+++ b/pkg/analysis_server_plugin/lib/registry.dart
@@ -7,6 +7,8 @@
 import 'package:analyzer/src/lint/linter.dart';
 
 abstract class PluginRegistry {
+  /// Register this fix [generator] for the given lint [code] with the
+  /// analyzer's rule registry.
   void registerFixForRule(LintCode code, ProducerGenerator generator);
 
   /// Register this [rule] with the analyzer's rule registry.