Improve documentation of ledger-based validation options in db-analyser

Flags `--validate-all-blocks` and `--minimum-block-validation` can cause confusion among `db-analyser` users who might think that they refer to /Ledger-based/ validation. In addition, this tool's documentation does not mention which option to use to only validate blocks in the ChainDB. We should therefore:

- [ ] Document the `--trace-ledger` option, which performs ledger-based validation of blocks.
- [ ] Add an example showing the usage of `--trace-ledger`, since this is a common enough use case.
- [x] Document `--validate-all-blocks` and `--minimum-block-validation` options to avoid confusion with `--trace-ledger` option. #1155
- [ ] Add progress tracing to the `--trace-ledger` analysis.