Clarify GNU vs MSVC for Windows

[Livven]

Describe the problem you are trying to solve

The book says:

On Windows, download and run the [rustup-init.exe built for i686-pc-windows-gnu target][setup]. In general, this is the build of rustup one should install on Windows. Despite being built against the GNU toolchain, the Windows build of rustup will install Rust for the MSVC toolchain if it detects that MSVC is installed.

However, the official Rust download page links to the MSVC version of rustup-init, going against the recommendation from the book.

Additionally, I ran the GNU version of rustup-init in a clean mcr.microsoft.com/windows:1809 Docker container, which does not have MSVC, as confirmed by the fact that cargo build tells me I am missing the linker, and yet I still get an MSVC toolchain by default.

So it seems that the book is wrong about the MSVC detection, or it does not work reliably.

Describe the solution you'd like

To me, it seems the correct solution would be to recommend the MSVC version of rustup-init, but in any case the recommendation from the book and official download link should probably be aligned.

Notes

See above.

[rbtcollins]

I believe the book to be out of date: we decided to recommend the MSVC built rustup a long time ago. See #1353