Clarify the seq-reduce documentation

* doc/lispref/sequences.texi (Sequence Functions): Ditto.

* lisp/emacs-lisp/seq.el (seq-reduce): Clarify the order of the
arguments (bug#43995).
1 job for emacs-27 in 102 minutes and 59 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Test
passed #8273
test-all

01:42:59