We should document that double-clicking rustup-init may not work properly

Problem

On Windows, if you run rustup-init.exe by double clicking the executable, it wouldn't detect custom paths in CARGO_HOME and RUSTUP_HOME and install to the default path.

Steps

1. Set the variables CARGO_HOME and RUSTUP_HOME via command line (setx ... /m) or Windows GUI
2. Double click the rustup-init.exe executable
3. Answer Y to the initial prompt regarding having to install additional dependencies
4. On the next screen, observe the incorrect paths being presented
5. Proceed with the installation. Choose default installation
6. Observe the absence of the installed app in the paths described by CARGO_HOME and RUSTUP_HOME
7. Observe the app being located in the default location (%USERPROFILE%\.cargo\ and %USERPROFILE%\.rustup\ respectively)

Workaround

1. Open a separate cmd instance
2. Navigate to the rustup-init.exe file and run it
3. Observe correct paths being displayed
4. Observe the app being installed into CARGO_HOME and RUSTUP_HOME properly

Notes

Windows version: Version 10.0.19042 Build 19042

Review the existing Windows installation documentation for rustup-init.exe and the reproduction steps in this issue. Document that double-clicking may ignore CARGO_HOME and RUSTUP_HOME, while running the executable from cmd respects them; verify the wording against both launch methods.