2021: Update for prelude changes (RFC 3114)

[ehuss]

I think the prelude documentation needs to be updated for the new edition behavior. See RFC 3114 and the edition guide chapter for more. IIUC, all the references to v1 in https://github.com/rust-lang/reference/blob/master/src/names/preludes.md need to be updated to the new model where the prelude depends on the edition. See also standard_library_imports.rs where this is implemented.