#9957: document that SpooledTemporaryFile.truncate does not take size arg

Doc/library/tempfile.rst

@@ -76,7 +76,8 @@ The module defines the following user-callable items:
    data is spooled in memory until the file size exceeds *max_size*, or
    until the file's :func:`fileno` method is called, at which point the
    contents are written to disk and operation proceeds as with
-   :func:`TemporaryFile`.
+   :func:`TemporaryFile`.  Also, it's ``truncate`` method does not
+   accept a ``size`` argument.
 
    The resulting file has one additional method, :func:`rollover`, which
    causes the file to roll over to an on-disk file regardless of its size.