clarify docs for mac x86 users

[pdg744]

During the installation process for x86_64-apple-darwin users end up being presented with the fact that cargo binstall cargo-risczero currently does not have prebuilt binaries for this architecture.

Users in this architecture should instead run cargo risczero build-toolchain, i.e. build the toolchain from scratch. This is different from what we mention in cargo-risczero's documentation, where we say we do provide prebuilt binaries.

We should be consistent across our documentation pages.

[linear]

ZKVM-398 clarify docs for mac x86 users

[pdg744]

apologies if some of that is incoherent -- I was taking notes based on oral comments.

[Cardosaum]

@pdg744 , I've updated the description of this issue.

There were other points mentioned there as well but I'll break them apart and create one GitHub issue for each of them.

[flaub]

We just need to remove this line:
https://github.com/risc0/risc0/blob/main/risc0/cargo-risczero/README.md?plain=1#L41