* lisp/isearch.el (isearch-message-prefix): Show "Multi-file" and
"Multi-buffer" instead of "Multi".

* lisp/misearch.el (multi-isearch-file-list): Autoload
multi-isearch-buffer-list and multi-isearch-file-list.
(multi-isearch-end): Reset multi-isearch-buffer-list and
multi-isearch-file-list to nil.

* doc/emacs/search.texi (Other Repeating Search): Add documentation for
multi-isearch-files and multi-isearch-files-regexp.