docs: three corrections to the Cassandra guides #658

Three small corrections to the Cassandra guides, found while reading #412 and kept out of that PR so it can ship. Line numbers are against #412 at 183903b.

1. docs/cassandra-setup.md line 24: -p 9042:9042 publishes CQL on every host interface (docker shows 0.0.0.0:9042), and line 60 says the node listens on 127.0.0.1. Until step 3 the node has no authentication, and after it the default cassandra superuser is still there. -p 127.0.0.1:9042:9042 keeps it local.

2. docs/getting-started.md line 84: on the authenticated node the guide requires, this cqlsh call stops with Connection timeout after 10s because it passes no credentials. The flags from the Cassandra section further down fix it.

3. docs/cassandra-setup.md line 210: there is no access key at this point in the guide, so the command fails with Unable to locate credentials, and a key ExtendDB did not issue gets UnrecognizedClientException. A pointer to the create-access-key steps in getting-started.md before this step fixes it.