crypto/tls: update CBC Lucky13 warning a bit

Some countermeasures were implemented in https://golang.org/cl/18130

Updates #13385

Change-Id: I723e1e3be0fa6d13767b65b145d90c89e92b2774
Reviewed-on: https://go-review.googlesource.com/33665
Reviewed-by: Adam Langley <agl@golang.org>

tls.go

@@ -5,7 +5,7 @@
 // Package tls partially implements TLS 1.2, as specified in RFC 5246.
 package tls
 
-// BUG(agl): The crypto/tls package does not implement countermeasures
+// BUG(agl): The crypto/tls package only implements some countermeasures
 // against Lucky13 attacks on CBC-mode encryption. See
 // http://www.isg.rhul.ac.uk/tls/TLStiming.pdf and
 // https://www.imperialviolet.org/2013/02/04/luckythirteen.html.