Merge #14364: doc: Clarify -blocksdir usage

ccc27bdcd2 doc: Clarify -blocksdir usage (Daniel McNally)

Pull request description:

  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.

ACKs for commit ccc27b:
  hebasto:
    utACK ccc27bdcd2d91fe2c023ad004019d5b970f21dbf

Tree-SHA512: 7b65f66b0579fd56e8c8cd4f9f22d6af56181817762a68deccd7fca51820ad82d9a0c48f5f1f012e746c67bcdae7af4555fad867cb620a9ca538d465c9d86c2b

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