(Help): Mention existence of Emacs and stand-alone Info at the very

(Help): Mention existence of Emacs and stand-alone Info at the very
beginning of the tutorial.
(Help-Inv): New node.
(Help-]): New node.
(Help-M): Systematically point out the differences between default
Emacs and stand-alone versions.  Delete second menu.
(Help-Xref): Systematically point out the differences between default
Emacs and stand-alone versions.
(Help-Int): Change `l' example.
(Expert Info): Fix typos.
(Emacs Info Variables): Mention `Info-hide-note-references' and new
default for `Info-scroll-prefer-subnodes'.