doc: add documentation for AtExit hook

Fixes: nodejs#999
PR-URL: nodejs#1014
Reviewed-By: Ben Noordhuis <[email protected]>

doc/api/addons.markdown

@@ -811,3 +811,71 @@ Test it with:
     var result = addon.add(obj1, obj2);
 
     console.log(result); // 30
+
+### AtExit hooks
+#### void AtExit(callback, args)
+
+* `callback`: `void (*)(void*)` - A pointer to the function to call at exit.
+* `args`: `void*` - A pointer to pass to the callback at exit.
+
+Registers exit hooks that run after the event loop has ended, but before the VM
+is killed.
+
+Callbacks are run in last-in, first-out order. AtExit takes two parameters:
+a pointer to a callback function to run at exit, and a pointer to untyped
+context data to be passed to that callback.
+
+The file `addon.cc` implements AtExit below:
+
+    // addon.cc
+    #undef NDEBUG
+    #include <assert.h>
+    #include <stdlib.h>
+    #include <node.h>
+
+    namespace demo {
+
+    using node::AtExit;
+    using v8::HandleScope;
+    using v8::Isolate;
+    using v8::Local;
+    using v8::Object;
+
+    static char cookie[] = "yum yum";
+    static int at_exit_cb1_called = 0;
+    static int at_exit_cb2_called = 0;
+
+    static void at_exit_cb1(void* arg) {
+      Isolate* isolate = static_cast<Isolate*>(arg);
+      HandleScope scope(isolate);
+      Local<Object> obj = Object::New(isolate);
+      assert(!obj.IsEmpty()); // assert VM is still alive
+      assert(obj->IsObject());
+      at_exit_cb1_called++;
+    }
+
+    static void at_exit_cb2(void* arg) {
+      assert(arg == static_cast<void*>(cookie));
+      at_exit_cb2_called++;
+    }
+
+    static void sanity_check(void*) {
+      assert(at_exit_cb1_called == 1);
+      assert(at_exit_cb2_called == 2);
+    }
+
+    void init(Local<Object> exports) {
+      AtExit(sanity_check);
+      AtExit(at_exit_cb2, cookie);
+      AtExit(at_exit_cb2, cookie);
+      AtExit(at_exit_cb1, exports->GetIsolate());
+    }
+
+    NODE_MODULE(addon, init);
+
+    }  // namespace demo
+
+Test in JavaScript by running:
+
+    // test.js
+    var addon = require('./build/Release/addon');