crypto/tls: update CBC Lucky13 warning a bit
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 <db68fba232b2c49ed3e9d3081df349dde7b61dfd@golang.org>

Jigsaw-Code/utls