bug#28584: 24.5; Emacs manual: add index entries for `multi-isearch-*' commands

[Drew Adams <drew.adams@oracle.com>]

In the Emacs manual, if you use `i multi' then, even if you have a way
to substring-match index entries, you see nothing about using Isearch
across multiple files or buffers.

There are these index entries, for example:

  `multiple-file search and replace'
  `search and replace in multiple files'

But those just take you to node `Tags Search'.

Please consider doc changes, to (1) include more doc for Isearching
across multiple files and buffers, (2) index that doc, and (3)
cross-reference between that doc and node `Tags Search'.

The existing index entries mentioned above no doubt need to be changed,
to distinguish tags-based searching from Isearching.

For #1: Please document the `multi-isearch-*' commands more thoroughly.
Currently, `multi-isearch-files', and `multi-isearch-files-regexp' are
not mentioned.


In GNU Emacs 24.5.1 (i686-pc-mingw32)
 of 2015-04-11 on LEG570
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --prefix=/c/usr --host=i686-pc-mingw32'