[Top App Bar] Specify that either AppBarLayout or MaterialToolbar should be themed or transparent in the doc

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**Is your feature request related to a problem? Please describe.**
The doc doesn't say any about the relationship of `AppBarLayout` and `MaterialToolbar`, but in their code I found that only one of them should be themed (taking an `style` property in the layout, and you have to theme one of them (otherwise the appbar could be very strange).
Moreover, the in collapsible layout, only the `AppBarLayout`'s style will take effect.

**Describe the solution you'd like**
In the doc, add instructions about their relationship and what should take a `style` property under each circumstance.

Start with the Top App Bar documentation and inspect the AppBarLayout and MaterialToolbar guidance and code behavior described in the issue. Clarify which component should receive a style in each layout circumstance, including collapsible layouts, and mark the work done when the documentation explains the relationship and styling choices.

Written by the indexing model from the issue text.