[winforms] Document the Application type, specifically, the DoEvents object

The `Application` type in WinForms isn't documented. `DoEvents` is a commonly used method to unblock the UI thread, however, it should be used careful and people frequently use it when there are better alternatives to choose from. `DoEvents` should be documented.

Reference articles:
- https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.application.doevents?view=windowsdesktop-7.0
- https://stackoverflow.com/questions/11352301/how-to-use-doevents-without-being-evil