Make DateTime::__construct()'s changelog clearer on the point that exceptions
are thrown if the date format is invalid, rather than the date itself.

Fixes doc bug #64908 (DateTime's constructor accepts time that not exists
because of DST).


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@330341 c90b9560-bf6c-de11-be94-00142212c4b1