_content: document cgo requirement for race detector

The race detector doc doesn't state cgo,
the content is basically as ALTree suggested in issue 51235.

Fixes golang/go#51235

Change-Id: I03356c01f4542e69a02c712063afb77ae68cc90f
Reviewed-on: https://go-review.googlesource.com/c/website/+/386454
Reviewed-by: DO NOT USE <[email protected]>
Trust: Dmitri Shuralyov <[email protected]>

Author: mengzhuo

_content/doc/articles/race_detector.html

@@ -410,10 +410,10 @@ <h3 id="Unsynchronized_send_and_close_operations">Unsynchronized send and close
 close(c)
 </pre>
 
-<h2 id="Supported_Systems">Supported Systems</h2>
+<h2 id="Requirements">Requirements</h2>
 
 <p>
-  The race detector runs on
+  The race detector requires cgo to be enabled and supports
   <code>linux/amd64</code>, <code>linux/ppc64le</code>,
   <code>linux/arm64</code>, <code>freebsd/amd64</code>,
   <code>netbsd/amd64</code>, <code>darwin/amd64</code>,