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.

Apri xml/System.Windows.Data/UpdateSourceTrigger.xml alla voce collegata intorno alla riga 105 e confronta la formulazione con la panoramica collegata sul focus di WPF. Aggiorna la descrizione in modo che LostFocus faccia esplicitamente riferimento al focus logico, quindi verifica che la documentazione generata preservi tale distinzione.

Scritto dal modello di indicizzazione a partire dal testo della issue.