cmd/compile: update documentation for ONAME node with nil Func

cmd/compile: update documentation for ONAME node with nil Func

After CL 436435 chain, the only case left where we create an ONAME node
with nil Func is interface method from imported package.

Change-Id: I9d9144916d01712283f2b116973f88965715fea3
Reviewed-on: https://go-review.googlesource.com/c/go/+/468816
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>