godoc: show example function doc comments in UI

R=gri
CC=golang-dev
https://golang.org/cl/5677061

lib/godoc/example.html

@@ -4,11 +4,12 @@
 	</div>
 	<div class="expanded">
 		<p class="exampleHeading">▾ Example{{example_suffix .Name}}</p>
+		{{with .Doc}}<p>{{html .}}</p>{{end}}
 		<p>Code:</p>
 		<pre class="code">{{.Code}}</pre>
-		{{if .Output}}
+		{{with .Output}}
 		<p>Output:</p>
-		<pre class="output">{{html .Output}}</pre>
+		<pre class="output">{{html .}}</pre>
 		{{end}}
 	</div>
 </div>

src/cmd/godoc/godoc.go

@@ -539,8 +539,8 @@ func example_htmlFunc(funcName string, examples []*ast.Example, fset *token.File
 		}
 
 		err := exampleHTML.Execute(&buf, struct {
-			Name, Code, Output string
-		}{eg.Name, code, out})
+			Name, Doc, Code, Output string
+		}{eg.Name, eg.Doc, code, out})
 		if err != nil {
 			log.Print(err)
 		}

src/pkg/go/ast/example.go

@@ -16,6 +16,7 @@ import (
 
 type Example struct {
 	Name     string // name of the item being exemplified
+	Doc      string // example function doc string
 	Code     Node
 	Comments []*CommentGroup
 	Output   string // expected output
@@ -45,8 +46,13 @@ func Examples(files ...*File) []*Example {
 			if !isTest(name, "Example") {
 				continue
 			}
+			var doc string
+			if f.Doc != nil {
+				doc = f.Doc.Text()
+			}
 			flist = append(flist, &Example{
 				Name:     name[len("Example"):],
+				Doc:      doc,
 				Code:     f.Body,
 				Comments: file.Comments,
 				Output:   exampleOutput(f, file.Comments),