cmd/compile: describe how Go maps to wasm implementation

cmd/compile: describe how Go maps to wasm implementation

Change-Id: Ie4d8e1ae9c4c6046d27a27a61ef1147bc0ff373c
Reviewed-on: https://go-review.googlesource.com/c/go/+/455715
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Richard Musiol <neelance@gmail.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>