cmd/compile: add more doc for Name.Defn

cmd/compile: add more doc for Name.Defn

As discussion in CL 313289, Name.Defn is also be set for variables
declared in type switch, or name function.

Change-Id: I3deb3d79fca269356e4432b77df7c7720a523674
Reviewed-on: https://go-review.googlesource.com/c/go/+/313889
Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Trust: Dan Scales <danscales@google.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dan Scales <danscales@google.com>