Updated documentation with details on rmiUrl

It appears that you cannot pass  `rmiUrls` in the `cacheManagerPeerProviderFactory` definition. Instead you must specify using multiple calls to `rmiUrl` like so:

```groovy
cacheManagerPeerProviderFactory {
  factoryType 'rmi'
  rmiUrl '//server1:40001'
  rmiUrl '//server2:40001'
}
```

This updates the documentation with regards to distributed DSL.