Document that immutable folder discovery is recursive

## Why
The immutable folder discovery is recursive, but this behavior is not well documented or tested.

## What
Document and test the recursive folder discovery behavior.

## How
- [x] Document the function with the recursive behavior in the `mithril-cardano-node-internal-database`
- [x] Test the recursive behavior of the discovery in `mithril-cardano-node-internal-database`