Better docstring and parameter name for seq-find

* lisp/emacs-lisp/seq.el (seq-find): Improve the docstring and rename
the parameter `sentinel' to `default'.

* doc/lispref/sequences.texi (Sequence Functions): Update the
  documentation for `seq-find' accordingly.

lisp/emacs-lisp/seq.el

@@ -270,18 +270,18 @@ If so, return the non-nil value returned by PRED."
           (throw 'seq--break result))))
     nil))
 
-(cl-defgeneric seq-find (pred seq &optional sentinel)
+(cl-defgeneric seq-find (pred seq &optional default)
   "Return the first element for which (PRED element) is non-nil in SEQ.
-If no element is found, return SENTINEL or nil.
+If no element is found, return DEFAULT.
 
-Note that `seq-find' has an ambiguity if the found element is nil
-and if no SENTINEL is specified, as it cannot be known if an
-element was found or not."
+Note that `seq-find' has an ambiguity if the found element is
+identical to DEFAULT, as it cannot be known if an element was
+found or not."
   (catch 'seq--break
     (seq-doseq (elt seq)
       (when (funcall pred elt)
         (throw 'seq--break elt)))
-    sentinel))
+    default))
 
 (cl-defgeneric seq-count (pred seq)
   "Return the number of elements for which (PRED element) is non-nil in SEQ."