Cleanups to the ImageMagick code and docstrings.

Cleanups to the ImageMagick code and docstrings.

* lisp/image-mode.el (image-toggle-display-image): Signal an error if
not in Image mode.
(image-transform-mode, image-transform-resize)
(image-transform-set-rotation): Doc fix.
(image-transform-set-resize): Deleted.
(image-transform-set-scale, image-transform-fit-to-height)
(image-transform-fit-to-width): Handle image-toggle-display-image
and image-transform-resize directly.

* src/image.c (Fimagemagick_types): Doc fix, and comment cleanup.