doc: explain how to set GOPATH to a custom value

Updates #18294.

Change-Id: Ib6b84243a15ed921cc8960e5fa355fd7594181e6
Reviewed-on: https://go-review.googlesource.com/34821Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/code.html

@@ -160,9 +160,13 @@ $ <b>export GOPATH=$(go env GOPATH)</b>
 </pre>
 
 <p>
-To learn more about setting up the <code>GOPATH</code> environment variable,
-please see
-<a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>'go help gopath'</code></a>
+To learn more about the <code>GOPATH</code> environment variable, see
+<a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>'go help gopath'</code></a>.
+</p>
+
+<p>
+To use a custom workspace location,
+<a href="https://golang.org/wiki/EnvVariables#gopath">set the <code>GOPATH</code> environment variable</a>.
 </p>
 
 <h3 id="ImportPaths">Import paths</h3>