doc: fix errors and warnings with Texinfo 5

doc: fix errors and warnings with Texinfo 5

Or rather, with the development version 4.13.90, which will eventually
become Texinfo 5.0.

* doc/coreutils.texi: Use '@item' instead of '@itemx' in several places,
as Texinfo 5 refuses to process an '@itemx' that is not preceded by an
'@item'.  Ensure that node extended names in menus and sectioning are
consistent, and that ordering and presence of nodes in menus and in the
actual text are consistent as well.

Fixes http://bugs.gnu.org/11828