Clarify disabled-segment selection errors in the SegmentedButton documentation

The SegmentedButton documentation does not explain in the SelectedRowIndex property description that assigning the index of a disabled segment raises InvalidArgumentException, even in SelectionMode.Single. This behaviour is mentioned beneath the Methods table, where it is easy to miss.

Document the exception directly alongside SelectedRowIndex in both the Desktop and Web references. Make the same restriction clear for SetRowSelected so developers can find it when looking up either selection API.