curl.1: Document weaknesses in SSLv2 and SSLv3

Acknowledge that SSLv3 is also widely considered to be insecure.

Also, provide references for people who want to know more about why it's
insecure.

docs/curl.1

@@ -171,10 +171,11 @@ a level of control).
 .IP "-2, --sslv2"
 (SSL) Forces curl to use SSL version 2 when negotiating with a remote SSL
 server. Sometimes curl is built without SSLv2 support. SSLv2 is widely
-considered insecure.
+considered insecure (see RFC 6176).
 .IP "-3, --sslv3"
 (SSL) Forces curl to use SSL version 3 when negotiating with a remote SSL
-server. Sometimes curl is built without SSLv3 support.
+server. Sometimes curl is built without SSLv3 support. SSLv3 is widely
+considered insecure (see RFC 7568).
 .IP "-4, --ipv4"
 This option tells curl to resolve names to IPv4 addresses only, and not for
 example try IPv6.