doc: Revised DIR/MRC replication section in user guide.

- Adapted examples to latest changes in default configuration flies.
- Added example how to specify multiple DIR replicas at the client.

git-svn-id: https://xtreemfs.googlecode.com/svn/trunk@3497 f8528748-c2c0-11dd-ba5c-d3b7c2b3b534