Add more references to Emscripten and WASI

Author: juntyr

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

@@ -3,17 +3,25 @@
 **Tier: 2**
 
 The `wasm32-unknown-emscripten` target is a WebAssembly compilation target which
-uses the Emscripten compiler toolchain. Unlike the
+uses the [Emscripten](https://emscripten.org/) compiler toolchain. Unlike the
 [`wasm32-unknown-unknown`](./wasm32-unknown-unknown.md) target, which only has
 partial support for the Rust standard library, `wasm32-unknown-emscripten`
 integrates with JavaScipt on the web to provide a more featureful standard library.
 This target is often used when compiling for the web, accessing web APIs, or
 interoperating with C and C++ code.
 
-One existing user of this target (please feel free to edit and expand this list
-too) is the [`pyodide` project](https://github.com/pyodide/pyodide) which provides
-a Python runtime in WebAssembly using Emscripten and compiles Python extension
-modules written in Rust to the `wasm32-unknown-emscripten` target.
+One existing user of this target is the
+[`pyodide` project](https://github.com/pyodide/pyodide) which provides a Python
+runtime in WebAssembly using Emscripten and compiles Python extension modules
+written in Rust to the `wasm32-unknown-emscripten` target.
+
+If you want to target the web but no not need interop with C/C++ or you want to
+produce a bare-bones WebAssembly binary and you do not require access to the host
+environment (e.g. stdio, file access, networking, etc.), consider using the
+[`wasm32-unknown-unknown`](./wasm32-unknown-unknown) target instead.
+
+If you need access to the host environment, consider using one of the WASI targets
+([`wasm32-wasip1`](./wasm32-wasip1) or [`wasm32-wasip2`](./wasm32-wasip2)).
 
 ## Target maintainers
 
@@ -28,9 +36,10 @@ said, those interested in maintaining this target are:
 
 ## Requirements
 
-This target is cross-compiled. The Emscripten compiler toolchain must be
+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.
+code. Please refer to https://emscripten.org/docs/getting_started/downloads.html
+on how to get started with the emsdk.
 
 ## Building the target