rx: Use longest match for all-string 'or' forms (bug#37659)
Revert to the Emacs 26 semantics that always gave the longest match for rx 'or' forms with only string arguments. This guarantee was never well documented, but it is useful and people likely have come to rely on it. For example, prior to this change, (rx (or ">" ">=")) matched ">" even if the text contained ">=". * lisp/emacs-lisp/rx.el (rx--translate-or): Don't tell regexp-opt to preserve the matching order. * doc/lispref/searching.texi (Rx Constructs): Document the longest-match guarantee for all-string 'or' forms. * test/lisp/emacs-lisp/rx-tests.el (rx-or): Update test.