doc: document GOAMD64 environment variable

Update #47694

Change-Id: I9c90bd251616cd4d10434bd3b6e6c30c5c819e24
Reviewed-on: https://go-review.googlesource.com/c/go/+/363661
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.18.html

@@ -40,6 +40,19 @@ <h3 id="freebsd">FreeBSD</h3>
   FreeBSD 13.0+ will require a kernel with the COMPAT_FREEBSD12 option set (this is the default).
 </p>
 
+<h3 id="amd64">AMD64</h3>
+
+<p><!-- CL 349595 -->
+  Go 1.18 introduces the new <code>GOAMD64</code> environment variable which selects
+  a version of the AMD64 architecture. Allowed values are <code>v1</code>,
+  <code>v2</code>, <code>v3</code>, or <code>v4</code>. Each higher level requires,
+  and takes advantage of, additional processor features.  A detailed description of the
+  versions is <a href="https://en.wikipedia.org/wiki/X86-64#Microarchitecture_levels">here</a>.
+</p>
+<p>
+  The <code>GOAMD64</code> environment variable defaults to <code>v1</code>.
+</p>
+
 <h3 id="ppc64">PPC64</h3>
 
 <p><!-- CL 353969 -->
@@ -123,10 +136,6 @@ <h3 id="go-command">Go command</h3>
   third-party tools that need to collect package source code.)
 </p>
 
-<p><!-- CL 349595 -->
-  TODO: <a href="https://golang.org/cl/349595">https://golang.org/cl/349595</a>: cmd/go: add GOAMD64 environment variable
-</p>
-
 <h3 id="gofmt"><code>gofmt</code></h3>
 
 <p><!-- https://golang.org/issue/43566 -->