Improve the documentation of extension points

pkg/analysis_server/lib/analysis/navigation_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.analysis.navigation_core;
 
 import 'package:analysis_server/src/protocol.dart'
     show ElementKind, Location, NavigationRegion, NavigationTarget;
 import 'package:analyzer/src/generated/engine.dart' show AnalysisContext;
 import 'package:analyzer/src/generated/source.dart' show Source;
 
 /**
  * An object used to produce navigation regions.
  *
  * Clients are expected to subtype this class when implementing plugins.
  */
 abstract class NavigationContributor {
   /**
-   * Contribute navigation regions for a part of the given [source] into the
-   * given [collector]. The part is specified by the [offset] and [length].
-   * The [context] can be used to get analysis results.
+   * Contribute navigation regions for a subset of the content of the given
+   * [source] into the given [collector]. The subset is specified by the
+   * [offset] and [length]. The [context] can be used to access analysis results.
    */
   void computeNavigation(NavigationCollector collector, AnalysisContext context,
       Source source, int offset, int length);
 }
 
 /**
  * An object that [NavigationContributor]s use to record navigation regions
  * into.
  *
  * Clients are not expected to subtype this class.
  */
 abstract class NavigationCollector {
   /**
    * Record a new navigation region with the given [offset] and [length] that
-   * should navigation to the given [targetLocation].
+   * should navigate to the given [targetLocation].
    */
   void addRegion(
       int offset, int length, ElementKind targetKind, Location targetLocation);
 }