Use `or' instead of `union' for charset union in rx

Use `or' instead of `union' for charset union in rx

Design change suggested by Stefan Monnier.

* doc/lispref/searching.texi (Rx Constructs):
* etc/NEWS: Document.
* lisp/emacs-lisp/rx.el (rx--translate-or): Detect charset arguments.
(rx--charset-p): New.
(rx--translate-not, rx--charset-intervals, rx--translate-union):
Change from `union' to `or'.
(rx--translate-form, rx--builtin-forms, rx): Remove `union'.
* test/lisp/emacs-lisp/rx-tests.el (rx-union, rx-def-in-union)
(rx-intersection): Rename tests and change `union' to `or' and `|'.