This property is used when your view is a UIScrollView or similar, like a UITableView.（主要对UIScrollView及其子类UITableView起作用，主要调节contentInset的值）
You want your table to start where the navigation bar ends, because you wont see the whole content if not, but at the same time you want your table to cover the whole screen when scrolling.
(automaticallyAdjustsScrollViewInsets 默认值为YES，会自动调整contentInset的top的值，使UITableView从NavigationBar的底部开始，并且UITableView向上滚动的时候，可以穿越导航条)
In that case, setting edgesForExtendedLayout to None won't work because your table will start scrolling where the navigation bar ends and it won‘t go behind it.
（通过设置edgesForExtendedLayout这个属性，达不到这个效果，因为，不能穿越导航条，仅仅是在导航条下面滚动）
This is when is set to NO:
This property is used when your view is a UIScrollView or similar, like a UITableView.（主要对UIScrollView及其子类UITableView起作用，主要调节contentInset的值）
You want your table to start where the navigation bar ends, because you wont see the whole content if not, but at the same time you want your table to cover the whole screen when scrolling.
(automaticallyAdjustsScrollViewInsets 默认值为YES，会自动调整contentInset的top的值，使UITableView从NavigationBar的底部开始，并且UITableView向上滚动的时候，可以穿越导航条)
In that case, setting edgesForExtendedLayout to None won't work because your table will start scrolling where the navigation bar ends and it won‘t go behind it.
（通过设置edgesForExtendedLayout这个属性，达不到这个效果，因为，不能穿越导航条，仅仅是在导航条下面滚动）
This is when is set to NO:
And YES (by default):
