[Form] Document DateType limitations

[webmozart]

DateType is limited to dates between Fri, 13 Dec 1901 20:45:54 GMT and Tue, 19 Jan 2038 03:14:07 GMT on 32bit systems when

• the option input is set to 'timestamp', OR
• the option widget is set to'single_text'

This is due to a limitation in PHP itself:

The valid range of a timestamp is typically from Fri, 13 Dec 1901 20:45:54 GMT to Tue, 19 Jan 2038 03:14:07 GMT. (These are the dates that correspond to the minimum and maximum values for a 32-bit signed integer). However, before PHP 5.1.0 this range was limited from 01-01-1970 to 19-01-2038 on some systems (e.g. Windows).

For other values of input and widget, dates outside this range work fine.

see symfony/symfony#5227