Document legacy and clarify bestfit parameters for Imagick::resizeImage

[lacatoire]

This PR clarifies two optional parameters of Imagick::resizeImage()

Fixes #4816

[jordikroon]

According to the changelog file the legacy property has also been added to a few other methods. Would you be able to implement these as well?

To be exact, this is the changelog of Imagick 3.4.0.

Imagick::adaptiveResizeImage, Imagick::cropThumbnailImage Imagick::resizeImage,
Imagick::scaleImage, and Imagick::thumbnailImage have all had a rounding bug fixed.
An additional parameter has been added to each of them, 'bool $legacy'. If legacy
is true, the calculations are done with the small rounding bug that existed in
Imagick before 3.4.0. If false, the calculations should produce the same results as
ImageMagick CLI does.

[jordikroon]

Thanks! Looks good to me. Unless someone objects.