Document PY_SSIZE_T_CLEAN for PyArg_ParseTuple.

Doc/c-api/arg.rst

@@ -32,7 +32,7 @@ variable(s) whose address should be passed.
    converted to C strings using the default encoding.  If this conversion fails, a
    :exc:`UnicodeError` is raised.
 
-``s#`` (string, Unicode or any read buffer compatible object) [const char \*, int]
+``s#`` (string, Unicode or any read buffer compatible object) [const char \*, int (or :ctype:`Py_ssize_t`, see below)]
    This variant on ``s`` stores into two C variables, the first one a pointer to a
    character string, the second one its length.  In this case the Python string may
    contain embedded null bytes.  Unicode objects pass back a pointer to the default
@@ -40,6 +40,11 @@ variable(s) whose address should be passed.
    other read-buffer compatible objects pass back a reference to the raw internal
    data representation.
 
+   Starting with Python 2.5 the type of the length argument can be
+   controlled by defining the macro :cmacro:`PY_SSIZE_T_CLEAN` before
+   including :file:`Python.h`.  If the macro is defined, length is a
+   :ctype:`Py_ssize_t` rather than an int.
+
 ``s*`` (string, Unicode, or any buffer compatible object) [Py_buffer \*]
   Similar to ``s#``, this code fills a Py_buffer structure provided by the caller.
   The buffer gets locked, so that the caller can subsequently use the buffer even