Document php_imagick_thumbnail_dimensions

[Danack]

There is a legacy function called php_imagick_thumbnail_dimensions that is used internally by a couple of Imagick functions e.g. Imagick::adaptiveResizeImage, Imagick::scaleImage, Imagick::thumbnailImage.

It would be great if someone went through the code and docuemented clearly what it is doing.

https://github.com/Imagick/imagick/blob/3998cfcf1e81b315b32efdbdd95548062463dac3/imagick_helpers.c#L175-L255

It would probably make sense to expose that function as a static function to userland, so that tests can be written for it in PHP.