Mention `showTimePicker` function be can be used to show dialog with the time picker in the `TimePickerDialog` docs (#131932)

fixes https://github.com/flutter/flutter/issues/131931

Every time I search for the time picker in the API docs I end up in the `TimePickerDialog` docs.  

We should link `showTimePicker` so it can be easier to reach it and mention it can be used directly to show a dialog with a time picker.