cmd/compile: improving the documentation of various fields and functions

This is only changes to comments, so should be fine to go into 1.17.

Change-Id: I01e28dc76b03fb3ca846d976f8ac84bc2acb2ea2
Reviewed-on: https://go-review.googlesource.com/c/go/+/318009


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>