Improve documentation of image-related commands

Improve documentation of image-related commands

* 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.