Document wasmexport and WASI reactor/library mode. Also document
that we now permit more types for wasmimport.
Fixes #65199.
Updates #66984.
For #68545.
Change-Id: Id26a8c9496650cd154daed679b82223af1848eea
Reviewed-on: https://go-review.googlesource.com/c/go/+/634036 Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>