ox: Remove `comment' special value for `org-export-with-creator'

ox: Remove `comment' special value for `org-export-with-creator'

* lisp/ox.el (org-export-with-creator): Change default value.

* lisp/ox-ascii.el (org-ascii-template):
* lisp/ox-beamer.el (org-beamer-template):
* lisp/ox-latex.el (org-latex-template):
* lisp/ox-odt.el (org-odt-template):
* lisp/ox-org.el (org-org-template):
* lisp/ox-texinfo.el (org-texinfo-template): Treat
  `org-export-with-creator' as a boolean.

* testing/lisp/test-ox.el (test-org-export/parse-option-keyword):
  Update test.

* doc/org.texi (Export settings): Update allowed values in
  `org-export-with-creator'.

* etc/ORG-NEWS: Signal change.

Special `comment' value isn't meaningful for all back-ends and is not
implemented in every back-end where it makes sense anyway.

It is possible to add a comment including creator at the end of the
document using a body filter instead.