Document imageflip() & imagecrop*() in the migration guide

# imagecrop*() will be documented shortly

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@329879 c90b9560-bf6c-de11-be94-00142212c4b1
Update the migration guide a little further, including WebP support

# WebP documentation will also come along shortly
# Affine & co support will be in Beta 2 and be documented hereafter once I can get to test it properly

git-svn-id: https://svn.php.net/repository/phpdoc/en/trunk@329880 c90b9560-bf6c-de11-be94-00142212c4b1