This adds documentation for BCardView, BCardLayout and BLayoutBuilder::Cards.
There is also a bit of cleanup for the BSplitView documentation.
It also makes explicit when a developer passes an invalid argument to
BCardLayout::SetVisibleItem(), by making that a debugger() call.
Change-Id: I17ac52cc773bb76c4f81beaa76f72af62a9e10f4
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2460
Reviewed-by: waddlesplash <waddlesplash@gmail.com>
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