Beef up the notes on the PHP 5.5 changes to pack() and unpack().

Specifically, I've added changelogs to the function pages (not sure how I
missed that to start with), expanded the discussion of what the changes are,
and moved them into the backward incompatible changes page, since they are.


git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@329132 c90b9560-bf6c-de11-be94-00142212c4b1