cmd/compile: update doc at top of iexport.go on the changes for typeparams

Add in the documentation of the new declarations and type descriptors to
support parameterized functions and types.

Change-Id: Ia21c544caa704309cc8fd639f104c192e1786b72
Reviewed-on: https://go-review.googlesource.com/c/go/+/350951


Trust: Dan Scales <danscales@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>