[Windows] Documentation for minor quirky edge case in Windows

Nobody has claimed this yet.

I think this is a very very minor edge case, but some may benefit from knowing this, so I decided to share.

- **Description:**
- OS: Windows (Windows 11)
- Luarocks version: 3.8.0 (Single Binary distribution, 64bit)
- Git Bash and cmd/PowerShell uses different paths for luarocks prefix (for config and installation).
- On Git Bash it's `~/.luarocks`
- On cmd/Powershell it's `%APPDATA%\luarocks`
- By default the two work as independent installations.
- **Important Quirk when trying to merge the two**
- If you keep `~/.luarocks` and make `%APPDATA%\luarocks` a link with `mklink /j ...` then when launching luarocks in PowerShell it wipes the hard link and makes a new `%APPDATA%\luarocks` directory.
- Therefore I personally recommend to keep `%APPDATA%\luarocks` and use a symbolic link for `~/.luarocks` with `ln -s ...`
- Other shells may not exhibit this behavior. For instance, `busybox sh` uses `%APPDATA%`

No documentation file or test is named in the issue. Review the existing Windows and configuration documentation, then add the reported differences between Git Bash and cmd/PowerShell paths and the link behavior; done means the edge case and recommended link direction are clearly documented.

Written by the indexing model from the issue text.