* lisp/image.el (image-map): Make it inherit from
'image-slice-map' instead of repeating the bindings.
* doc/emacs/files.texi (Image Mode): Document the key bindings set
by 'insert-image'. Add indexing.
* doc/lispref/display.texi (Showing Images): Make the description
of user commands more concise. Add index entries and
cross-reference to the Emacs manual.
* etc/NEWS: Rearrange entries relevant to image commands.