cmd/godoc: only show package documentation for commands

Fixed package.txt and adjusted package.html to match
structure (swapped if branches).

Fixes #4861.

R=golang-dev, adg, rsc
CC=golang-dev
https://golang.org/cl/7714043

lib/godoc/package.html

@@ -10,7 +10,11 @@
 	correspond to Go identifiers).
 -->
 {{with .PDoc}}
-	{{if not $.IsMain}}
+	{{if $.IsMain}}
+		{{/* command documentation */}}
+		{{comment_html .Doc}}
+	{{else}}
+		{{/* package documentation */}}
 		<div id="short-nav">
 			<dl>
 			<dd><code>import "{{html .ImportPath}}"</code></dd>
@@ -160,8 +164,6 @@
 				{{example_html $name $.Examples $.FSet}}
 			{{end}}
 		{{end}}
-	{{else}}  {{/* not a package; is a command */}}
-		{{comment_html .Doc}}
 	{{end}}
 
 	{{with $.Notes}}

lib/godoc/package.txt

@@ -2,14 +2,15 @@
 
 ---------------------------------------
 
-*/}}{{with .PDoc}}{{if not $.IsMain}}PACKAGE
+*/}}{{with .PDoc}}{{if $.IsMain}}COMMAND DOCUMENTATION
+
+{{comment_text .Doc "    " "\t"}}
+{{else}}PACKAGE DOCUMENTATION
 
 package {{.Name}}
     import "{{.ImportPath}}"
 
-{{else}}COMMAND DOCUMENTATION
-
-{{end}}{{comment_text .Doc "    " "\t"}}
+{{comment_text .Doc "    " "\t"}}
 {{example_text "" $.Examples $.FSet "    "}}{{/*
 
 ---------------------------------------
@@ -58,7 +59,7 @@ TYPES
 {{end}}{{range .Methods}}{{node .Decl $.FSet}}
 {{comment_text .Doc "    " "\t"}}
 {{$name := printf "%s_%s" $tname .Name}}{{example_text $name $.Examples $.FSet "    "}}
-{{end}}{{end}}{{end}}{{/*
+{{end}}{{end}}{{end}}{{end}}{{/*
 
 ---------------------------------------