(Introduction): Reword to make consistent with printed version only.

(Introduction): Reword to make consistent with printed version only.
(MS-DOS): Renamed from "MS-DOG".  All references updated.
(Daylight Savings): Remove this node: it is an exact duplicate of its
name-sake in calendar.texi.
<Top level>: Remove the body of all chapters and move them to the
new *-xtra.texi files.  Use @raisesections and @lowersections to
convert sections to chapters etc.