Address @harrishancock's review comments.

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

@@ -157,6 +157,9 @@ void UnixEventPort::ChildSet::checkExits() {
 }
 
 Promise<int> UnixEventPort::onChildExit(Maybe<pid_t>& pid) {
+  KJ_REQUIRE(capturedChildExit,
+      "must call UnixEventPort::captureChildExit() to use onChildExit().");
+
   ChildSet* cs;
   KJ_IF_MAYBE(c, childSet) {
     cs = *c;

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

@@ -127,7 +127,7 @@ public:
   //   which child, and therefore it may inadvertently reap children created by other threads.
 
   static void captureChildExit();
-  // Arranges for child process exit to be captured and handled via UnixEventPort, so than you may
+  // Arranges for child process exit to be captured and handled via UnixEventPort, so that you may
   // call `onChildExit()`. Much like `captureSignal()`, this static method must be called early on
   // in program startup.
   //