Add a x-ref to newer calling APIs.

Doc/api/abstract.tex

@@ -260,6 +260,8 @@ determination.
   result of the call on success, or \NULL{} on failure.  This is the
   equivalent of the Python expression \samp{apply(\var{callable},
   \var{args})} or \samp{\var{callable}(*\var{args})}.
+  Note that if you only pass \ctype{PyObject *} args,
+  \cfunction{PyObject_CallFunctionObjArgs} is a faster alternative.
   \bifuncindex{apply}
 \end{cfuncdesc}
 
@@ -274,6 +276,8 @@ determination.
   indicating that no arguments are provided. Returns the result of the
   call on success, or \NULL{} on failure.  This is the equivalent of
   the Python expression \samp{\var{o}.\var{method}(\var{args})}.
+  Note that if you only pass \ctype{PyObject *} args,
+  \cfunction{PyObject_CallMethodObjArgs} is a faster alternative.
 \end{cfuncdesc}