go/doc cleanup: remove some unnecessary string conversions

R=rsc
CC=golang-dev
https://golang.org/cl/642041

src/pkg/go/doc/doc.go

@@ -292,10 +292,9 @@ func (doc *docReader) addFile(src *ast.File) {
 	// collect BUG(...) comments
 	for _, c := range src.Comments {
 		text := c.List[0].Text
-		cstr := string(text)
-		if m := bug_markers.ExecuteString(cstr); len(m) > 0 {
+		if m := bug_markers.Execute(text); len(m) > 0 {
 			// found a BUG comment; maybe empty
-			if bstr := cstr[m[1]:]; bug_content.MatchString(bstr) {
+			if btxt := text[m[1]:]; bug_content.Match(btxt) {
 				// non-empty BUG comment; collect comment without BUG prefix
 				list := copyCommentList(c.List)
 				list[0].Text = text[m[1]:]