go1.5.txt: better checking of struct tags

Change-Id: I0e726a2c0fa1231011b183c3e917414d876555d8
Reviewed-on: https://go-review.googlesource.com/2700Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.txt

@@ -8,6 +8,9 @@ log: add global Output function (https://golang.org/cl/2686)
 net/http: support for setting trailers from a server Handler (https://golang.org/cl/2157)
 net/smtp: add TLSConnectionState accessor (https://golang.org/cl/2151)
 
+Tools:
+
+cmd/vet: better validation of struct tags (https://golang.org/cl/2685)
 
 Performance: