Document cl-n... set operations consistently

Document cl-n... set operations consistently

The docstrings of cl-nintersection and cl-nset-difference have been
inconsistent with their manual entries since the beginning of
emacs.git history (bug#76017).  This patch settles on the weaker and
thus backward-compatible requirement that only their first argument
be safe to mutate.

* lisp/emacs-lisp/bytecomp.el: Include only first argument in
mutates-arguments property.
* lisp/emacs-lisp/cl-seq.el (cl-nintersection, cl-nset-difference):
Make docstring consistent with manual in that the second argument is
not modified.
* test/lisp/emacs-lisp/cl-seq-tests.el (cl-nintersection-test)
(cl-nset-difference-test): Simplify.
(cl-nset-difference): Pass fresh list as second argument, otherwise
destructive modifications to it could go undetected.

(cherry picked from commit ac143186c04ffd729cfe11abd99f02abdf742f64)