Clarify the return value of PyObject_IsInstance().

Doc/api/abstract.tex

@@ -129,14 +129,15 @@ for which they do not apply, they will raise a Python exception.
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{int}{PyObject_IsInstance}{PyObject *inst, PyObject *cls}
-  Return \code{1} if \var{inst} is an instance of the class \var{cls}
-  or a subclass of \var{cls}.  If \var{cls} is a type object rather
-  than a class object, \cfunction{PyObject_IsInstance()} returns
-  \code{1} if \var{inst} is of type \var{cls}.  If \var{inst} is not a
-  class instance and \var{cls} is neither a type object or class
-  object, \var{inst} must have a \member{__class__} attribute --- the
-  class relationship of the value of that attribute with \var{cls}
-  will be used to determine the result of this function.
+  Returns \code{1} if \var{inst} is an instance of the class \var{cls}
+  or a subclass of \var{cls}, or \code{0} if not.  On error, returns
+  \code{-1} and sets an exception.  If \var{cls} is a type object
+  rather than a class object, \cfunction{PyObject_IsInstance()}
+  returns \code{1} if \var{inst} is of type \var{cls}.  If \var{inst}
+  is not a class instance and \var{cls} is neither a type object or
+  class object, \var{inst} must have a \member{__class__} attribute
+  --- the class relationship of the value of that attribute with
+  \var{cls} will be used to determine the result of this function.
   \versionadded{2.1}
 \end{cfuncdesc}