doc: add note about missing lock in sample code

The sample code in 'Interfaces and methods' section contains a
data race. Handlers are served concurrently. The handler does write
and read operations; `go test -race` would fail (with concurrent
requests). Since the doc is frozen and the code remains less
cluttered without locks/atomic, don't change the sample code.

Change-Id: I654b324d2f0b7f48497822751907c7d39e2f0e3d
Reviewed-on: https://go-review.googlesource.com/c/go/+/239877
Reviewed-by: Rob Pike <r@golang.org>

doc/effective_go.html

@@ -2336,10 +2336,9 @@ of the request from the client.
 </p>
 <p>
 For brevity, let's ignore POSTs and assume HTTP requests are always
-GETs; that simplification does not affect the way the handlers are
+GETs; that simplification does not affect the way the handlers are set up.
-set up.  Here's a trivial but complete implementation of a handler to
+Here's a trivial implementation of a handler to count the number of times
-count the number of times the
+the page is visited.
-page is visited.
 </p>
 <pre>
 // Simple counter server.
@@ -2355,6 +2354,11 @@ func (ctr *Counter) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 <p>
 (Keeping with our theme, note how <code>Fprintf</code> can print to an
 <code>http.ResponseWriter</code>.)
+In a real server, access to <code>ctr.n</code> would need protection from
+concurrent access.
+See the <code>sync</code> and <code>atomic</code> packages for suggestions.
+</p>
+<p>
 For reference, here's how to attach such a server to a node on the URL tree.
 </p>
 <pre>