Clarify the default-boundp doc string

* src/data.c (Fdefault_boundp): Doc string clarification (bug#44141).

src/data.c

@@ -1639,8 +1639,9 @@ default_value (Lisp_Object symbol)
 
 DEFUN ("default-boundp", Fdefault_boundp, Sdefault_boundp, 1, 1, 0,
        doc: /* Return t if SYMBOL has a non-void default value.
-This is the value that is seen in buffers that do not have their own values
-for this variable.  */)
+A variable may have a buffer-local or a `let'-bound local value.  This
+function says whether the variable has a non-void value outside of the
+current context.  Also see `default-value'.  */)
   (Lisp_Object symbol)
 {
   register Lisp_Object value;