Add `unmatchable' as alias for (or) in rx (bug#37659)

* lisp/emacs-lisp/rx.el (rx--translate-symbol, rx--builtin-symbols, rx):
* test/lisp/emacs-lisp/rx-tests.el (rx-atoms):
* doc/lispref/searching.texi (Rx Constructs):
* etc/NEWS:
Add `unmatchable', more descriptive than (or), and corresponding to
the variable `regexp-unmatchable'.

test/lisp/emacs-lisp/rx-tests.el

@@ -186,6 +186,8 @@
 (ert-deftest rx-atoms ()
   (should (equal (rx anychar anything)
                  "\\(?:.\\|\n\\)\\(?:.\\|\n\\)"))
+  (should (equal (rx unmatchable)
+                 "\\`a\\`"))
   (should (equal (rx line-start not-newline nonl any line-end)
                  "^...$"))
   (should (equal (rx bol string-start string-end buffer-start buffer-end