Add example to documentation of a DateInput with input_type='date'

[dennisvang]

Rationale

By default, the forms.widgets.DateInput widget renders as a simple text field.

However, a "date picker", like ​the one implemented in contrib.admin is often much more convenient.

Fortunately it is now possible to take advantage of the browser's built-in date picker, using the HTML ​<input type='date'> element.

The only problem is that, although the default DateInput can be easily modified to use the <input type='date'> element, this may not be so obvious to people unfamiliar with the inner workings of Django (e.g. ​forms.widgets.Input.input_type).

Moreover, some people may not even know the HTML type='date' exists, nor that ​browsers have built-in date pickers.

Request

I believe it would be very helpful if the documentation included an example, like these two provided by Tim Graham in ticket #33100:

class DateInput(forms.DateInput):
    input_type = 'date'

or

DateInput(attrs={'type': 'date'})

This has the additional advantage of documenting the use of the forms.widgets.Input.input_type attribute, which ​appears in the documentation but is not mentioned explicitly in the text, as far as I see.

If necessary, I would be willing to prepare a pull request for this.