doc: shred: Correct docs on default number of overwrites.

doc: shred: Correct docs on default number of overwrites.

* NEWS: Mention the change to the default number of passes.
* doc/Makefile.am: Update constants.texi with the default
number of passes shred uses, so that the documentation
will automatically reflect any future changes.
* doc/coreutils.texi (shred invocation): Update the description
of the --iterations option to have the correct default number,
while still conveying that there are 25 internal patterns
that may be useful.