Document cross-environment support (Apple Silicon Rosetta x64 emulation)

[wolfv]

Problem description

When you run a pixi.toml that is only compatible with osx-64 it works fine on osx-arm64 automatically (just a little warning about using rosetta).

We should document this.

Also pixi global install --platform=osx-64 works (with some caveats at this point – pixi global update-all doesn't work nicely).