ox-texinfo: Fix location of text before first headline
* doc/org.texi (Info directory file): Fix node name.
(Headings and sectioning structure): Document location of text before
first headline.
(A Texinfo example): Update example.
* lisp/ox-texinfo.el (org-texinfo-template): Handle contents of very
first section.
(org-texinfo-section): Ignore very first section.
According to Texinfo specifications, Top node contents should not
appear in printed output. Move it near "@top" command, within
"@ifnottex".