Issue #24952: Clarify default argument of stack_size() in threading, thread

Patch from Mattip.
diff --git a/Doc/library/thread.rst b/Doc/library/thread.rst
index 8a625f8..f1cee24 100644
--- a/Doc/library/thread.rst
+++ b/Doc/library/thread.rst
@@ -98,7 +98,8 @@
    Return the thread stack size used when creating new threads.  The optional
    *size* argument specifies the stack size to be used for subsequently created
    threads, and must be 0 (use platform or configured default) or a positive
-   integer value of at least 32,768 (32kB). If changing the thread stack size is
+   integer value of at least 32,768 (32kB). If *size* is not specified,
+   0 is used. If changing the thread stack size is
    unsupported, the :exc:`error` exception is raised.  If the specified stack size is
    invalid, a :exc:`ValueError` is raised and the stack size is unmodified.  32kB
    is currently the minimum supported stack size value to guarantee sufficient