doc: improve documentation for GOAMD64

The change clarifies in the release notes for go1.18 that the variable
is for compilation and references the microarchitecture description in
the Go Wiki,
https://golang.org/wiki/MinimumRequirements#amd64,
and references the same information in the output of go help
environment.

Fixes #50174

Change-Id: I6a7d5a06f48463a810c96cc9c76fe66113d5147c
Reviewed-on: https://go-review.googlesource.com/c/go/+/372194


Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>