cmd/compile: document go:wasmimport directive

Fixes golang#64972

Change-Id: I784a8728afbdef9736681d56f0a79c82a00609d6
GitHub-Last-Rev: bf158db
GitHub-Pull-Request: golang#64991
Reviewed-on: https://go-review.googlesource.com/c/go/+/554455
Reviewed-by: Achille Roussel <achille.roussel@gmail.com>
Reviewed-by: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/doc.go

@@ -295,5 +295,27 @@ The declaration of lower.f may also have a linkname directive with a
 single argument, f. This is optional, but helps alert the reader that
 the function is accessed from outside the package.
 
+	//go:wasmimport importmodule importname
+
+The //go:wasmimport directive is wasm-only and must be followed by a
+function declaration.
+It specifies that the function is provided by a wasm module identified
+by ``importmodule`` and ``importname``.
+
+	//go:wasmimport a_module f
+	func g()
+
+The types of parameters and return values to the Go function are translated to
+Wasm according to the following table:
+
+    Go types        Wasm types
+    int32, uint32   i32
+    int64, uint64   i64
+    float32         f32
+    float64         f64
+    unsafe.Pointer  i32
+
+Any other parameter types are disallowed by the compiler.
+
 */
 package main