Document immutables folder discovery is recursive (part2)

## Why
The immutable folder discovery is recursive, but not compatible in the aggragator with the snapshotter.

**snapshot_all_completed_immutables** function will try to produce archive by reconstructing a path using root db folder, immutable foder, and filenames, so it won't work if we have folders between given path and immutable directory (see [compressed_archive_snapshotter.rs lign 63](https://github.com/input-output-hk/mithril/blob/main/mithril-aggregator/src/services/snapshotter/compressed_archive_snapshotter.rs#L63) )

## What (to groom)
ideas :
- fix the path construction in the snapshot_all_completed_immutables function

## How (to groom)
- [ ] Document the command options using the recursive behavior :
- [ ] in the client
- [ ] in the aggregator
- [ ] in the signer