Clarify ABI incompatibility flags

Author: juntyr

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

@@ -96,17 +96,23 @@ This target can be cross-compiled from any host.
 ## Emscripten ABI Compatibility
 
 The Emscripten compiler toolchain does not follow a semantic versioning scheme
-that clearly indicates when breaking changes to the ABI can be made. If you are
-using the pre-compiled `std` from rustup, it may be compiled with a different
-Emscripten version than the local Emscripten you have installed. If a breaking
-change to the ABI was made in between the two versions, your code will exhibit
-undefined behaviour. If you come across issues, you can rebuild the Rust standard
-library with your local Emscripten version using
+that clearly indicates when breaking changes to the ABI can be made. Additionally,
+Emscripten offers many different ABIs even for a single version of Emscripten
+depending on the linker flags used, e.g. `-fexceptions` and `-sWASM_BIGINT`. If
+the ABIs mismatch, your code may exhibit undefined behaviour.
+
+To ensure that the ABIs of your Rust code, of the Rust standard library, and of
+other code compiled for Emscripten all match, you should rebuild the Rust standard
+library with your local Emscripten version and settings using:
 
 ```sh
 cargo +nightly -Zbuild-std build
 ```
 
+If you still want to use the pre-compiled `std` from rustup, you should ensure
+that your local Emscripten matches the version used by Rust and be careful about
+any `-C link-arg`s that you compiled your Rust code with.
+
 ## Testing
 
 This target is not extensively tested in CI for the rust-lang/rust repository. It
@@ -156,3 +162,8 @@ Note that Rust code compiled for `wasm32-unknown-emscripten` currently enables
 `-fexceptions` (JS exceptions) by default unless the Rust code is compiled with
 `-Cpanic=abort`. `-fwasm-exceptions` (WASM exceptions) is not yet currently supported,
 see <https://github.com/rust-lang/rust/issues/112195>.
+
+Please refer to the [Emscripten ABI compatibility](#emscripten-abi-compatibility)
+section to ensure that the features that are enabled do not cause an ABI mismatch
+between your Rust code, the pre-compiled Rust standard library, and other code compiled
+for Emscripten.