Clean up wording of client usage expectations

pkg/analysis_server/lib/plugin/analysis/occurrences/occurrences_core.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analysis_server.plugin.analysis.occurrences.occurrences_core;
 
 import 'package:analysis_server/plugin/protocol/protocol.dart'
     show Element, Occurrences;
 import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
 import 'package:analyzer/src/generated/source.dart' show Source;
 
 /**
  * An object that [OccurrencesContributor]s use to record occurrences into.
  *
- * Clients are not expected to subtype this class.
+ * Clients may not extend, implement or mix-in this class.
  */
 abstract class OccurrencesCollector {
   /**
    * Record a new element occurrences.
    */
   void addOccurrences(Occurrences occurrences);
 }
 
 /**
  * An object used to produce occurrences.
  *
- * Clients are expected to subtype this class when implementing plugins.
+ * Clients may implement this class when implementing plugins.
  */
 abstract class OccurrencesContributor {
   /**
    * Contribute occurrences into the given [collector].
    * The [context] can be used to get analysis results.
    */
   void computeOccurrences(
       OccurrencesCollector collector, AnalysisContext context, Source source);
 }