Clarify docs for inspect.getargspec() that the fourth value is None when there

are no default arguments for the function.

Doc/lib/libinspect.tex

@@ -255,9 +255,9 @@ Note:
   \var{args} is a list of the argument names (it may contain nested lists).
   \var{varargs} and \var{varkw} are the names of the \code{*} and
   \code{**} arguments or \code{None}.
-  \var{defaults} is a tuple of default argument values; if this tuple
-  has \var{n} elements, they correspond to the last \var{n} elements
-  listed in \var{args}.
+  \var{defaults} is a tuple of default argument values or None if there are no
+  default arguments; if this tuple has \var{n} elements, they correspond to
+  the last \var{n} elements listed in \var{args}.
 \end{funcdesc}
 
 \begin{funcdesc}{getargvalues}{frame}