GetMessage用法错误导致程序不能退出

#include <windows.h>
#include <stdio.h>
LRESULT CALLBACK WinUoowProc( //回调函数声明
HWND hwnd,
UINT uMsg,
WPARAM wParam,
LPARAM lParam
);
int WINAPI WinMain( HINSTANCE hInstance, //WinMain主函数
HINSTANCE hPrevInstance,
LPSTR lpCmdLine,
int nCmdShow
)
{
WNDCLASS wndcls; // 创建窗口类
wndcls.cbClsExtra=0;
wndcls.cbWndExtra=0;
wndcls.hbrBackground=(HBRUSH)GetStockObject(BLACK_BRUSH);
wndcls.hCursor=LoadCursor(NULL,IDC_CROSS);
wndcls.hIcon=LoadIcon(NULL,IDI_ERROR);
wndcls.hInstance=hInstance;
wndcls.lpfnWndProc=WinUoowProc;
wndcls.lpszClassName="First Window";
wndcls.lpszMenuName=NULL;
wndcls.style=CS_DBLCLKS |CS_HREDRAW | CS_VREDRAW;
RegisterClass(&wndcls); //注册窗口
HWND hwnd; //创建窗口
hwnd=CreateWindow("First Window","Window BY Dancer_dus7",WS_OVERLAPPEDWINDOW,
300,200,600,400,NULL,NULL,hInstance,NULL);
ShowWindow(hwnd,SW_SHOWNORMAL); //显示窗口
UpdateWindow(hwnd); //更新窗口
MSG msg;
while(GetMessage(&msg,NULL/*hwnd*/,0,0)) //主循环 注意若注释处改成hwnd，则程序仍运行在任务管理器
{
TranslateMessage(&msg);
DispatchMessage(&msg);
}
return 0;
}
LRESULT CALLBACK WinUoowProc( //回调函数
HWND hwnd,
UINT uMsg,
WPARAM wParam,
LPARAM lParam
)
{
switch(uMsg) //消息处理
{
case WM_CHAR:
char szChar[20];
sprintf(szChar,"char is %d",wParam);
MessageBox(hwnd,szChar,"key code:",0);
break;
case WM_LBUTTONDOWN:
MessageBox(hwnd,"mouse clicked!","mouse",MB_OK);
HDC hdc;
hdc=GetDC(hwnd);
TextOut(hdc,0,50,"HELLO WORLD!",strlen("HELLO WORLD!"));
ReleaseDC(hwnd,hdc);
break;
case WM_PAINT:
HDC hDC;
PAINTSTRUCT ps;
hDC=BeginPaint(hwnd,&ps);
TextOut(hDC,0,0,"First Window!",strlen("First Window!"));
EndPaint(hwnd,&ps);
break;
case WM_DESTROY:
PostQuitMessage(0);
break;
default:
return DefWindowProc(hwnd,uMsg,wParam,lParam);
}
return 0;
}

MSDN中的说明，要避免while(GetMessage( lpMsg, hWnd, 0, 0))这种用法，避免出现-1的结果，窗口已经销毁，但是仍在运行。
Note that the function return value can be nonzero, zero, or -1. Thus, you should avoid code like this:

(GetMessage( lpMsg, hWnd, 0, 0))...

The possibility of a -1 return value means that such code can lead to fatal application errors.

while(GetMessage( lpMsg, NULL, 0, 0))才正确。

但是觉得还是没有说清楚，就是为什么GetMessage第二个参数为hwnd的时候就会出现窗口销毁而程序仍在任务管理窗口运行的现象，msdn好像没有说的很明白？望高手解答。