strconv: clarify doc for Atoi return type

Change-Id: I47bd98509663d75b0d4dedbdb778e803d90053cf
Reviewed-on: https://go-review.googlesource.com/24216Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/strconv/atoi.go

@@ -197,7 +197,7 @@ func ParseInt(s string, base int, bitSize int) (i int64, err error) {
 	return n, nil
 }
 
-// Atoi is shorthand for ParseInt(s, 10, 0).
+// Atoi returns the result of ParseInt(s, 10, 0) converted to type int.
 func Atoi(s string) (int, error) {
 	i64, err := ParseInt(s, 10, 0)
 	return int(i64), err