This PR attempts to clarify and correct the `-blocksdir` argument description and default value. `-blocksdir` does not refer to the full path to the actual `blocks` directory, but rather the root/parent directory which contains the `blocks` directory. Accordingly, the default value is `<datadir>` and not `<datadir>/blocks` - this behavior of defaulting to the datadir can also be seen in init.cpp:

```cpp
    if (gArgs.IsArgSet("-blocksdir")) {
        path = fs::system_complete(gArgs.GetArg("-blocksdir", ""));
        if (!fs::is_directory(path)) {
            path = "";
            return path;
        }
    } else {
        path = GetDataDir(false);
    }
```

It also attempts to clarify that only the `.dat` files containing block data are impacted by `-blocksdir`, not the index files.

I believe this would close #12828.

hebasto:
  utACK ccc27bdcd2d91fe2c023ad004019d5b970f21dbf