cmd/compile/internal/noder: add method wrapper comments

cmd/compile/internal/noder: add method wrapper comments

Add commentary explaining why and how we create method wrappers.

Change-Id: Idf35a77d0483f24c2163e11f5e001fd5536cca63
Reviewed-on: https://go-review.googlesource.com/c/go/+/558395
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>