Add 'read-extended-command-predicate'

* doc/emacs/m-x.texi (M-x): Document it.
* doc/lispref/commands.texi (Interactive Call): Document it further.

* lisp/simple.el (read-extended-command-predicate): New user option.
(read-extended-command-predicate): Use it.
(completion-in-mode-p): New function (the default predicate).

lisp/emacs-lisp/seq.el

@@ -455,6 +455,7 @@ negative integer or 0, nil is returned."
         (setq sequence (seq-drop sequence n)))
       (nreverse result))))
 
+;;;###autoload
 (cl-defgeneric seq-intersection (sequence1 sequence2 &optional testfn)
   "Return a list of the elements that appear in both SEQUENCE1 and SEQUENCE2.
 Equality is defined by TESTFN if non-nil or by `equal' if nil."