Add notes that UnixEventPort doesn't work after fork().

c++/src/kj/async-io.h

@@ -477,6 +477,12 @@ AsyncIoContext setupAsyncIo();
 //       print(text);
 //       return 0;
 //     }
+//
+// WARNING: An AsyncIoContext can only be used in the thread and process that created it. In
+//   particular, note that after a fork(), an AsyncIoContext created in the parent process will
+//   not work correctly in the child, even if the parent ceases to use its copy. In particular
+//   note that this means that server processes which daemonize themselves at startup must wait
+//   until after daemonization to create an AsyncIoContext.
 
 // =======================================================================================
 // inline implementation details

c++/src/kj/async-unix.h

@@ -51,6 +51,12 @@ class UnixEventPort: public EventPort {
   // The implementation reserves a signal for internal use.  By default, it uses SIGUSR1.  If you
   // need to use SIGUSR1 for something else, you must offer a different signal by calling
   // setReservedSignal() at startup.
+  //
+  // WARNING: A UnixEventPort can only be used in the thread and process that created it. In
+  //   particular, note that after a fork(), a UnixEventPort created in the parent process will
+  //   not work correctly in the child, even if the parent ceases to use its copy. In particular
+  //   note that this means that server processes which daemonize themselves at startup must wait
+  //   until after daemonization to create a UnixEventPort.
 
 public:
   UnixEventPort();