(define-key-after): Doc fix.

lisp/subr.el

@@ -217,8 +217,8 @@ This is like `define-key' except that the binding for KEY is placed
 just after the binding for the event AFTER, instead of at the beginning
 of the map.
 The order matters when the keymap is used as a menu.
-KEY must contain just one event type--it must be a string or vector
-of length 1."
+KEY must contain just one event type--that is to say, it must be
+a string or vector of length 1."
   (or (keymapp keymap)
       (signal 'wrong-type-argument (list 'keymapp keymap)))
   (if (> (length key) 1)