doc: tls: clarify server cipher list

* Make it clear that ECDHE-RSA-AES128-SHA256 and AES128-GCM-SHA256 are
  TLS v1.2 ciphers.

* Note that RC4 is under suspicion.
doc: tls: note that SSLv2 is disabled by default

As of commit 39aa894, SSLv2 support is disabled by default.  Update
the documentation to reflect that.