Extend target testing and building instructions

Co-authored-by: Hood Chatham <[email protected]>

Author: juntyr

src/doc/rustc/src/platform-support/wasm32-unknown-emscripten.md

@@ -40,9 +40,18 @@ If you are only targeting the web and need to access web APIs, the
 ## Requirements
 
 This target is cross-compiled. The Emscripten compiler toolchain (emsdk) must be
-installed to link Rust-compiled code with Emscripten's libc and other C/C++
-code. Please refer to <https://emscripten.org/docs/getting_started/downloads.html>
-on how to get started with the emsdk.
+installed to link WASM binaries with `emcc`. Please refer to
+<https://emscripten.org/docs/getting_started/downloads.html> on how to get started
+with the emsdk.
+
+You can also build the emsdk from source with:
+
+```sh
+git clone https://github.com/emscripten-core/emsdk.git --depth 1
+./emsdk/emsdk install 3.1.68
+./emsdk/emsdk activate 3.1.68
+source ./emsdk/emsdk_env.sh
+```
 
 ## Building the target
 
@@ -85,21 +94,38 @@ undefined behaviour. If you come across issues, you can rebuild the Rust standar
 library with your local Emscripten version using
 
 ```sh
-$ cargo +nightly -Zbuild-std build
+cargo +nightly -Zbuild-std build
 ```
 
 ## Testing
 
-This target is not extensively tested in CI for the rust-lang/rust repository.
-It's recommended to test the [`wasm32-wasip1`](./wasm32-wasip1.md) target instead
-for WebAssembly compatibility.
+This target is not extensively tested in CI for the rust-lang/rust repository. It
+can be tested locally, for example, with:
+
+```sh
+./x.py test --target wasm32-unknown-emscripten --skip src/tools/linkchecker
+```
+
+To run these tests, both `emcc` and `node` need to be in your `$PATH`. You can
+install `node`, for example, using `nvm` by following the instructions at
+<https://github.com/nvm-sh/nvm#install--update-script>.
+
+Still, it's recommended to test the [`wasm32-wasip1`](./wasm32-wasip1.md) target
+instead for WebAssembly compatibility.
 
 ## Conditionally compiling code
 
 It's recommended to conditionally compile code for this target with:
 
 ```text
-#[cfg(all(target_family = "wasm", target_os = "emscripten"))]
+#[cfg(target_os = "emscripten")]
+```
+
+It may sometimes be necessary to conditionally compile code for WASM targets
+which do *not* use emscripten, which can be achieved with:
+
+```text
+#[cfg(all(target_family = "wasm", not(target_os = "emscripten)))]
 ```
 
 ## Enabled WebAssembly features
@@ -108,7 +134,8 @@ WebAssembly is an evolving standard which adds new features such as new
 instructions over time. This target's default set of supported WebAssembly
 features will additionally change over time. The `wasm32-unknown-emscripten` target
 inherits the default settings of LLVM which typically, but not necessarily, matches
-the default settings of Emscripten as well.
+the default settings of Emscripten as well. At link time, `emcc` configures the
+linker to use Emscripten's settings.
 
 Please refer to the [`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md)
 target's documentation on which WebAssembly features Rust enables by default, how