doc: improve od --help and man page

doc: improve od --help and man page

* src/od.c: Redorder the information output by --help
to ease interpretation and so that appropriate sections
are generated by help2man.
* doc/coreutils.texi (od invocation): Fix an incorrect
reference to @var{n}, which should be @var{bytes}.
* man/od.x: Add an "Examples" section, and move the
default od format to there, and add a commonly required
format to generate hexdumps.
Reported by Akim Demaille in http://bugs.gnu.org/13280.