docs(user-guide): use brew install rustup instead of rustup-init

[rami3l]

Homebrew/homebrew-core#177983 is using brew install rustup now, so I'm adjusting the docs accordingly:

Please note that now you need to add $(brew --prefix rustup)/bin to PATH manually to reach to the rustup-installed toolchain. Additionally, you might run brew unlink rust to make sure you won't accidentally trigger the brew-managed rust toolchain.
#1236 (comment)

The original paragraph specific to brew-installed rustup-init has been moved and extended to a more general how-to description of the two methods being used by different package managers (thus not limited to apt and brew), so our users will hopefully have even fewer chances of getting lost.

[djc]

Looks great, thanks!