Document updated Praefect virtual storage configuration format

What does this MR do?

Praefect's virtual storage configuration format in omnibus left no room for other virtual storage related configuration values as the nodes were configured directly in the root of the virtual storage's configuration object. The format was changed to address this by moving the node configuration under the 'nodes' key. This commit updates the documentation to use the updated format.

The change was made in a backwards compatible manner. If nodes are configured in the root of the object, Omnibus corrects this by moving unknown keys under the 'nodes' key.

Related issues

omnibus-gitlab!4988 (merged)
omnibus-gitlab#5981 (closed)

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