cmd/internal/obj: clean up checkaddr

cmd/internal/obj: clean up checkaddr

Coalesce identical cases.
Give it a proper doc comment.
Fix comment locations.
Update/delete old comments.

Passes toolstash-check -all.

Change-Id: I88d9cf20e6e04b0c1c6583e92cd96335831f183f
Reviewed-on: https://go-review.googlesource.com/38442
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Cherry Zhang <cherryyz@google.com>