Fix hover documentation for parameters.

pkg/analysis_server/lib/src/computer/computer_hover.dart

 // Copyright (c) 2014, 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 computer.hover;
 
 import 'package:analysis_server/src/protocol.dart' show HoverInformation;
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 
@@ skipping 93 matching lines @@
             hover.containingClassDescription = containingClass.toString();
           }
           // containing library
           LibraryElement library = element.library;
           if (library != null) {
             hover.containingLibraryName = library.name;
             hover.containingLibraryPath = library.source.fullName;
           }
         }
         // documentation
-        String dartDoc = element.computeDocumentationComment();
-        dartDoc = _removeDartDocDelimiters(dartDoc);
-        hover.dartdoc = dartDoc;
+        hover.dartdoc = _computeDocumentation(element);
       }
       // parameter
       hover.parameter = _safeToString(expression.bestParameterElement);
       // types
       hover.staticType = _safeToString(expression.staticType);
       hover.propagatedType = _safeToString(expression.propagatedType);
       // done
       return hover;
     }
     // not an expression
     return null;
   }
 
+  String _computeDocumentation(Element element) {
+    if (element is ParameterElement) {
+      element = element.enclosingElement;
+    }
+    String dartDoc = element.computeDocumentationComment();
+    return _removeDartDocDelimiters(dartDoc);
+  }
+
   static _safeToString(obj) => obj != null ? obj.toString() : null;
 }