[Docs]Update material menu's description about menu default overlap behavior on document

[waynewaynetsai]

By default, current version(^7.3.7)'s angular material menu won't overlap its trigger element.
If we want the menu overlap the trigger element, we need to set [overlapTrigger] to true manually.

It's default position behavior is opposite to the description on document.

[docs link] https://material.angular.io/components/menu/overview?fbclid=IwAR0VwYfp7HyiEJTFIgvHGcy-lPnqiXvx8xn1cB4XVDzynCqz6eZxR0J72OM

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