Change doc-view-mode-map prefix key 's' to 'c'.

Change doc-view-mode-map prefix key 's' to 'c'.

* doc/emacs/misc.texi (DocView Slicing): Change prefix key 's' to 'c'.

* lisp/doc-view.el (doc-view-mode-map): Change prefix key 's' to 'c'.

* lisp/image-mode.el (image-mode-map): Add image-transform-set-scale to menu.

* doc/emacs/files.texi (Image Mode): Describe commands
image-transform-fit-both, image-transform-set-scale, image-transform-reset.

* etc/NEWS: Rearrange image sections.

https://lists.gnu.org/archive/html/emacs-devel/2020-04/msg01315.html