cmd/compile: document go:wasmimport directive
Fixes #64972
Change-Id: I784a8728afbdef9736681d56f0a79c82a00609d6
GitHub-Last-Rev:
bf158dbeaaa495dc258c590c7a21cf9e25e76ee7
GitHub-Pull-Request: golang/go#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>