crypto/tls: improve documentation for ServerName.

Users of the low-level, Client function are frequenctly missing the
fact that, unless they pass a ServerName to the TLS connection then it
cannot verify the certificates against any name.

This change makes it clear that at least one of InsecureSkipVerify and
ServerName should always be set.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/65440043