C-h m in Info mode should mention "search" for Info-index

[Drew Adams]

`C-h m'  in Info mode mentions `i' (Info-index) with this explanation: "Look
up a topic in this file's Index and move to that node."

That's a pretty good description, but a newbie using `C-h m' and looking for
how to search the manual will not find it by searching for "search".
Although "look up" is perhaps technically more correct here, it might be
more helpful to say, "Search for a topic in this manual's Index and move to
it." Then, searchinf for "search" in `C-h m' will bring you to `s', `S', and
`i'. Otherwise, some users will use `s' when they could better use `i',
because they are unaware of the latter.

The proposed text also gets rid of a reference to "file" - it is not obvious
to a newbie what the "file" is all about here: "manual" is clearer.

It also gets rid of a reference to "node" - since it speaks of a "topic", it
is enough to say that it moves to that topic; "topic" is used here in two
senses: 1) an index entry and 2) the destination of that entry.