Improve docs for UpdateSourceTrigger.LostFocus

https://github.com/dotnet/dotnet-api-docs/blob/c9d0e3742d0e7efd9e41a5302f415de20bfa07b8/xml/System.Windows.Data/UpdateSourceTrigger.xml#L105

There are two different focus concepts in WPF (see [1](https://docs.microsoft.com/en-us/dotnet/desktop/wpf/advanced/focus-overview)). The docs don't specify which kind of focus this relates to. The docs should say `logical focus` and not just `focus`.

The lack of this one word has contributed to a lot of confusion. See [2](https://stackoverflow.com/questions/16916861/textbox-lostfocus-is-not-raised-when-clicking-on-certain-other-controls), [3](https://stackoverflow.com/questions/10208861/wpf-data-bound-tabcontrol-doesnt-commit-changes-when-new-tab-is-selected), [4](https://stackoverflow.com/questions/57493/wpf-databind-before-saving) etc.