date() documentation correction

The date() docs were incorrectly stating that the argument should be in a format supported by PHP’s [date()](http://php.net/manual/en/function.date.php) function. That’s true of the |date filter which is used to _format_ dates, but not the date() function, which is used to _create_ new dates.

I double-checked the code and whatever you pass into date() ultimately will get passed to a [DateTime constructor](http://php.net/manual/en/datetime.construct.php), which accepts an actual date string, formatted in one of PHP’s [date and time format](http://php.net/manual/en/datetime.formats.php), which is a different beast than the actual format definition string you might pass into PHP’s date() function.