Multi backend documentation is incomplete [JIRA: DOC-432]

[lukebakken]

From this email thread, with regard to the multi backend documentation:

Trying to use the memory backend via multi.
What I did in riak.conf:

storage_backend = multi

Then creating a bucket type:

riak-admin bucket-type create os_cache '{"props": {"write_once": true,"backend":"memory","n_val":2,"r":1,"w":1,"dw":1}}'
riak-admin bucket-type activate os_cache

Then I PUT an object:

curl -v -XPUT -H "Content-Type: application/octet-stream" 
'http://localhost:8098/types/os_cache/buckets/os_cache/keys/test1' -d 'test'

Next, I shut down all of the servers at once, leaving nothing in the cluster, then restarting them. And a new get for the above URL returns "test".
When I set storage_backend to memory, it works as expected, after a full cluster stop, the bucket is empty.

Here is a configuration that could have been used, that would create two backends, one using memory that is called my_memory_backend, and one using leveldb that is called my_leveldb_backend:

storage_backend = multi
multi_backend.my_memory_backend.storage_backend = memory
multi_backend.my_leveldb_backend.storage_backend = leveldb

Then, the rest of the configuration is as follows:

riak-admin bucket-type create os_cache '{"props": {"write_once": true,"backend":"my_memory_backend","n_val":2,"r":1,"w":1,"dw":1}}'
riak-admin bucket-type activate os_cache

This is somewhat related to issue #1548