Ch. 21: call out Chrome multiple-connections issue

src/ch21-01-single-threaded.md

@@ -109,8 +109,17 @@ It could also be that the browser is trying to connect to the server multiple
 times because the server isn’t responding with any data. When `stream` goes out
 of scope and is dropped at the end of the loop, the connection is closed as
 part of the `drop` implementation. Browsers sometimes deal with closed
-connections by retrying, because the problem might be temporary. The important
-factor is that we’ve successfully gotten a handle to a TCP connection!
+connections by retrying, because the problem might be temporary.
+
+Browsers also sometimes open multiple connections to the server without sending
+any requests, so that if they *do* later send requests, they can happen faster.
+When this happens, our server will see each connection, regardless of whether
+there are any requests over that connection. Many versions of Chrome-based
+browsers do this, for example; you can disable that optimization by using =
+private browsing mode or use a different browser.
+
+The important factor is that we’ve successfully gotten a handle to a TCP
+connection!
 
 Remember to stop the program by pressing <kbd>ctrl</kbd>-<kbd>c</kbd> when
 you’re done running a particular version of the code. Then restart the program