Rearrange nodes.

Rearrange nodes.
(Top): Update menu.  Change ref `Info for Experts' to
`Advanced Info Commands'.
(Getting Started): Fix description of manual's parts.
(Help-Int): Change xref `Info Search' to `Search Index', and
`Expert Info' to `Advanced'.
(Advanced): Move node one level up.
(Search Text, Search Index): New nodes split out from `Info Search'.
(Go to node, Choose menu subtopic, Create Info buffer): New nodes
split out from `Advanced'.
(Advanced, Emacs Info Variables): De-document editing an Info file
in Info.
(Emacs Info Variables): Move node from `Expert Info' to `Advanced'.
(Creating an Info File): Delete node and move its text to
`Expert Info'.