docs: update alternator on write isolation
docs: update alternator on write isolation

Docs are appended with information on write isolation - which levels
are implemented in alternator and how to configure them properly.

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla