[Docs] Detailed description of code example for collapsing toolbar style

[cworld1]

I think it is necessary to add the specific topAppBar primary key description code to the code instance of collapsing toolbar style. Although this hint appeared in the normal toolbar style, it wasn't obvious enough that it took me hours to figure out what the problem was. Setting layout_height to another option causes the component to not work properly and the title to not display properly. Hopefully this new description will help others avoid this problem.

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[cworld1]

Emmm... What's wrong about this pr?