godoc: show line numbers for non-go files (bug fix)

Also: Give line numbers a style and make them less intrusive.

R=rsc
CC=golang-dev
https://golang.org/cl/4060043

doc/all.css

@@ -157,6 +157,10 @@ h1#title {
 pre.ebnf, pre.grammar {
   background: #FFFFE0;
 }
+span.ln {
+  font-size: 80%;
+  color: #777777;
+}
 span.comment {
   color: #002090;
 }

src/cmd/godoc/format.go

@@ -360,12 +360,12 @@ func FormatText(text []byte, line int, goSource bool, pattern string, selection
 	if pattern != "" {
 		highlights = regexpSelection(text, pattern)
 	}
-	if comments != nil || highlights != nil || selection != nil {
+	if line >= 0 || comments != nil || highlights != nil || selection != nil {
 		var lineTag LinkWriter
 		if line >= 0 {
 			lineTag = func(w io.Writer, _ int, start bool) {
 				if start {
-					fmt.Fprintf(w, "<a id=\"L%d\"></a>%5d\t", line, line)
+					fmt.Fprintf(w, "<a id=\"L%d\"></a><span class=\"ln\">%6d</span>\t", line, line)
 					line++
 				}
 			}