Fix documentation and one misfeature of Texinfo mode update commands.

Fix documentation and one misfeature of Texinfo mode update commands.

 lisp/textmodes/texnfo-upd.el: Update commentary.  Add a warning not
 to use texinfo-update-node and commands that call it if the
 Texinfo file uses @node lines without next/prev/up pointers.
 (texinfo-all-menus-update, texinfo-master-menu)
 (texinfo-update-node, texinfo-every-node-update)
 (texinfo-multiple-files-update): Doc fix.  Warn against updating
 all the @node lines.
 (texinfo-master-menu): Only call texinfo-update-node if the prefix
 argument is numeric.  See also this thread:
 http://lists.gnu.org/archive/html/emacs-devel/2011-12/msg00156.html.