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"

doc/languages-frameworks/rust.section.md

@@ -15,7 +15,7 @@ For other versions such as daily builds (beta and nightly),
 use either `rustup` from nixpkgs (which will manage the rust installation in your home directory),
 or use a community maintained [Rust overlay](#using-community-rust-overlays).
 
-## Compiling Rust applications with Cargo {#compiling-rust-applications-with-cargo}
+## `buildRustPackage`: Compiling Rust applications with Cargo {#compiling-rust-applications-with-cargo}
 
 Rust applications are packaged by using the `buildRustPackage` helper from `rustPlatform`:
 
@@ -596,7 +596,7 @@ buildPythonPackage rec {
 }
 ```
 
-## Compiling Rust crates using Nix instead of Cargo {#compiling-rust-crates-using-nix-instead-of-cargo}
+## `buildRustCrate`: Compiling Rust crates using Nix instead of Cargo {#compiling-rust-crates-using-nix-instead-of-cargo}
 
 ### Simple operation {#simple-operation}