Follow-up r79845: add function documentation; only call wfMakeDirsParent if we have...

Follow-up r79845: add function documentation; only call wfMakeDirsParent if we have a dest path.

On r79845 I submitted a wrong commit summary, here is it for reference:

(bug 6672) Images are now autorotated according to their EXIF orientation. Following the Flickr example, this only affects thumbnails, with the source unrotated.

* Introduced BitmapHandler::canRotate() and BitmapHandler::getRotation() to check if rotation is supported and what the amount of rotation is
* Factored out scaler determination into BitmapHandler::getScalerType()
* ImageMagick uses the -auto-orientation option
* GD uses imagerotate()
* Unconditionally show the thumb size on the image page, don't know why this wasn't shown for SVGs etc.