improve `erigon snapshots reset --help`

have user-understandable description in `erigon snapshots reset --help`:
- what is this tool
- what it does
- which flags (now useful flags are at bottom)
- description of `--dry-run` says `Print files` but actually it doesn't print:
```
go run ./cmd/erigon seg reset --dry-run --datadir ~/data/chiado31_full
INFO[08-17|16:05:48.678] logging to file system log dir=/Users/alex/Library/Erigon/logs file prefix=erigon log level=info json=false
INFO[08-17|16:05:48.679] logging to file system log dir=/Users/alex/data/chiado31_full/logs file prefix=erigon log level=info json=false
INFO[08-17|16:05:48.680] [db] open label=chaindata sizeLimit=1TB pageSize=16KB
INFO[08-17|16:05:48.680] read chain name from config chain=chiado
INFO[08-17|16:05:48.680] Loading remote snapshot hashes
INFO[08-17|16:05:55.660] Loaded preverified snapshots hashes len=1259 chain=chiado
INFO[08-17|16:05:55.660] Resetting snapshots directory path=/Users/alex/data/chiado31_full/snapshots
INFO[08-17|16:05:55.724] Files NOT removed from snapshots directory torrents=1259 data=1259
INFO[08-17|16:05:55.724] Files removed from snapshots directory torrents=0 data=0
INFO[08-17|16:05:55.724] Removing chaindata dir path=/Users/alex/data/chiado31_full/chaindata
INFO[08-17|16:05:55.724] Removed snapshots lock file path=preverified.toml
INFO[08-17|16:05:55.724] Reset complete. Start Erigon as usual, missing files will be downloaded.
```
- any other DX/UX of `erigon snapshots reset` are highly-welcome