_content/doc/articles/race_detector: cgo requires a C compiler

Fixes golang/go#56937

Change-Id: Ia9aca2c8f57e5f5641f40712df00d932578d13a5
Reviewed-on: https://go-review.googlesource.com/c/website/+/453435
Reviewed-by: Bryan Mills <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>

Author: ianlancetaylor

_content/doc/articles/race_detector.html

@@ -413,7 +413,9 @@ <h3 id="Unsynchronized_send_and_close_operations">Unsynchronized send and close
 <h2 id="Requirements">Requirements</h2>
 
 <p>
-  The race detector requires cgo to be enabled and supports
+  The race detector requires cgo to be enabled, and on non-Darwin
+  systems requires an installed C compiler.
+  The race detector 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>,