strings: complete Reader doc string

Follow-up on https://golang.org/cl/247523.

Change-Id: I9e91a6d77271e640d84851f2e2a4c6d2150a2b43
Reviewed-on: https://go-review.googlesource.com/c/go/+/266438
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/strings/reader.go

@@ -10,8 +10,8 @@ import (
 	"unicode/utf8"
 )
 
-// A Reader implements the io.Reader, io.ReaderAt, io.Seeker, io.WriterTo,
-// io.ByteScanner, and io.RuneScanner interfaces by reading
+// A Reader implements the io.Reader, io.ReaderAt, io.ByteReader, io.ByteScanner,
+// io.RuneReader, io.RuneScanner, io.Seeker, and io.WriterTo interfaces by reading
 // from a string.
 // The zero value for Reader operates like a Reader of an empty string.
 type Reader struct {