encoding/xml: update docs for Token

Fixes #13757.

Change-Id: I1b52593df8df0e98ce7342767eb34eccecc11761
Reviewed-on: https://go-review.googlesource.com/18854
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/xml/xml.go

@@ -227,7 +227,8 @@ func NewDecoder(r io.Reader) *Decoder {
 //
 // Token guarantees that the StartElement and EndElement
 // tokens it returns are properly nested and matched:
-// if Token encounters an unexpected end element,
+// if Token encounters an unexpected end element
+// or EOF before all expected end elements,
 // it will return an error.
 //
 // Token implements XML name spaces as described by