doc/../rust.section.md: prominently mention `buildRustXX` names

This was a source of massive confusion for me when I first learned my way around nixpkgs' rust machinery.  I seek to save others from that confusion.

* `buildRustPackage` should have been named `buildRustPackageUsingCargo`

* `buildRustCrate` should have been named `buildRustPackageUsingNix`

It is, unfortunately, too late to fix this.  Let's do the next best thing and make the names `buildRustPackage` and `buildRustCrate` very prominent in the documentation, so readers see immediately that they need to learn the following jargon:

* `buildRustPackage` means "build this Rust crate by calling `cargo` in one (or two) monolithic derivations"

* `buildRustCrate` means "build this Rust crate by calling `rustc` in one derivation for each crate"

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