FWIW, I would find it useful if `Info-index` could be called from any
buffer, say to search the Elisp manual if I'm on a emacs-lisp-mode
buffer.  In fact, it might be handy to associate a list of Info nodes,
rather than a single node, to a buffer (this is how the "evdocs"
package works, see my recent message in a different thread).

`Info-index` won't find a needle in a haysack, but one advantage over
`info-apropos` is the possibility to do incremental filtering out of the
box, with any of the incremental completion frameworks.