regexp: name result parameters referenced from docs

Fixes #2953

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5653051

src/pkg/regexp/regexp.go

@@ -676,7 +676,7 @@ func (re *Regexp) FindString(s string) string {
 // location of the leftmost match in s of the regular expression.  The match
 // itself is at s[loc[0]:loc[1]].
 // A return value of nil indicates no match.
-func (re *Regexp) FindStringIndex(s string) []int {
+func (re *Regexp) FindStringIndex(s string) (loc []int) {
 	a := re.doExecute(nil, nil, s, 0, 2)
 	if a == nil {
 		return nil
@@ -688,7 +688,7 @@ func (re *Regexp) FindStringIndex(s string) []int {
 // location of the leftmost match of the regular expression in text read from
 // the RuneReader.  The match itself is at s[loc[0]:loc[1]].  A return
 // value of nil indicates no match.
-func (re *Regexp) FindReaderIndex(r io.RuneReader) []int {
+func (re *Regexp) FindReaderIndex(r io.RuneReader) (loc []int) {
 	a := re.doExecute(r, nil, "", 0, 2)
 	if a == nil {
 		return nil