Added documentation for PyObject_IsInstance() and PyObject_IsSubclass().

Doc/api/api.tex

@@ -1493,6 +1493,42 @@ the equivalent of the Python expression \samp{unistr(\var{o})}.
 Called by the \function{unistr()}\bifuncindex{unistr} built-in function.
 \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.
+\versionadded{2.1}
+\end{cfuncdesc}
+
+Subclass determination is done in a fairly straightforward way, but
+includes a wrinkle that implementors of extensions to the class system
+may want to be aware of.  If \class{A} and \class{B} are class
+objects, \class{B} is a subclass of \class{A} if it inherits from
+\class{A} either directly or indirectly.  If either is not a class
+object, a more general mechanism is used to determine the class
+relationship of the two objects.  When testing if \var{B} is a
+subclass of \var{A}, if \var{A} is \var{B},
+\cfunction{PyObject_IsSubclass()} returns true.  If \var{A} and
+\var{B} are different objects, \var{B}'s \member{__bases__} attribute
+is searched in a depth-first fashion for \var{A} --- the presence of
+the \member{__bases__} attribute is considered sufficient for this
+determination.
+
+\begin{cfuncdesc}{int}{PyObject_IsSubclass}{PyObject *derived,
+                                            PyObject *cls}
+Returns \code{1} if the class \var{derived} is identical to or derived
+from the class \var{cls}, otherwise returns \code{0}.  In case of an
+error, returns \code{-1}.  If either \var{derived} or \var{cls} is not
+an actual class object, this function uses the generic algorithm
+described above.
+\versionadded{2.1}
+\end{cfuncdesc}
+
 
 \begin{cfuncdesc}{int}{PyCallable_Check}{PyObject *o}
 Determine if the object \var{o} is callable.  Return \code{1} if the