Add gc header size to returned sizeof information

Doc/library/sys.rst

Index: Doc/library/sys.rst
===================================================================
--- Doc/library/sys.rst	(revision 64518)
+++ Doc/library/sys.rst	(working copy)
@@ -409,13 +409,22 @@
    :func:`setrecursionlimit`.
 
 
-.. function:: getsizeof(object)
+.. function:: getsizeof(object[, default][, gc_head=True])

[Martin v. Löwis, 2008/07/07 05:24:06]

I would like to reconsider the addition of the gc_head parameter. I still can't
understand why anybody would like to omit this memory usage from the size of the
object. If there is a real need for such a parameter, it can always be added
later; taking it back if not needed is more difficult.

 
    Return the size of an object in bytes. The object can be any type of
    object. All built-in objects will return correct results, but this
-   does not have to hold true for third-party extensions as it is implementation 
+   does not have to hold true for third-party extensions as it is implementation
    specific.
 
+   The *default* argument allows to define a value which will be returned
+   if the object type does not provide means to retrieve the size and would
+   cause a `TypeError`. 
+
+   func:`getsizeof` calls the object's __sizeof__ method and adds an additional
+   garbage collector overhead if the object is managed by the garbage collector.
+   The *gc_head* argument allows to turn off the inclusion of this overhead
+   in the returned result. 
+
    .. versionadded:: 2.6