Formalize that the Py_VISIT macro requires that the tp_traverse

implementation it's used in must give its arguments specific names.

Doc/api/newtypes.tex

@@ -1664,13 +1664,14 @@ The \member{tp_traverse} handler must have the following type:
 \end{ctypedesc}
 
 To simplify writing \member{tp_traverse} handlers, a
-\cfunction{Py_VISIT()} is provided:
+\cfunction{Py_VISIT()} macro is provided.  In order to use this macro,
+the \member{tp_traverse} implementation must name its arguments
+exactly \var{visit} and \var{arg}:
 
 \begin{cfuncdesc}{void}{Py_VISIT}{PyObject *o}
-  Call the \var{visit} for \var{o} with \var{arg}. If \var{visit}
-  returns a non-zero value, then return it.  Using this macro,
-  \member{tp_traverse} handlers look like:
-
+  Call the \var{visit} callback, with arguments \var{o} and \var{arg}.
+  If \var{visit} returns a non-zero value, then return it.  Using this
+  macro, \member{tp_traverse} handlers look like:
 
 \begin{verbatim}
 static int

Doc/ext/newtypes.tex

@@ -841,8 +841,8 @@ the extra argument \var{arg} passed to the traversal method.  It
 returns an integer value that must be returned if it is non-zero.
 
 
-Python 2.4 and higher provide a \cfunction{Py_VISIT()} that automates
-calling visit functions.  With \cfunction{Py_VISIT()}, the
+Python 2.4 and higher provide a \cfunction{Py_VISIT()} macro that automates
+calling visit functions.  With \cfunction{Py_VISIT()},
 \cfunction{Noddy_traverse()} can be simplified:
 
 
@@ -856,6 +856,11 @@ Noddy_traverse(Noddy *self, visitproc visit, void *arg)
 }
 \end{verbatim}
 
+\note{Note that the \member{tp_traverse} implementation must name its
+    arguments exactly \var{visit} and \var{arg} in order to use
+    \cfunction{Py_VISIT()}.  This is to encourage uniformity
+    across these boring implementations.}
+
 We also need to provide a method for clearing any subobjects that can
 participate in cycles.  We implement the method and reimplement the
 deallocator to use it:

Include/objimpl.h

@@ -302,7 +302,11 @@ PyAPI_FUNC(void) PyObject_GC_Del(void *);
 		( (type *) _PyObject_GC_NewVar((typeobj), (n)) )
 
 
-/* Utility macro to help write tp_traverse functions */
+/* Utility macro to help write tp_traverse functions.
+ * To use this macro, the tp_traverse function must name its arguments
+ * "visit" and "arg".  This is intended to keep tp_traverse functions
+ * looking as much alike as possible.
+ */
 #define Py_VISIT(op)					\
         do { 						\
                 if (op) {				\