bpo-34613: document the correct value of limit argument of asyncio.StreamReader (GH-9121)

The default value of asyncio.StreamReader *limit* is `_DEFAULT_LIMIT` instead of `None`.

<!-- issue-number: [bpo-34613](https://www.bugs.python.org/issue34613) -->
https://bugs.python.org/issue34613
<!-- /issue-number -->
(cherry picked from commit b4ec362)

Co-authored-by: Bram <cortex@worlddomination.be>

Author: miss-islington

Doc/library/asyncio-stream.rst

@@ -126,10 +126,12 @@ Stream functions
 StreamReader
 ============
 
-.. class:: StreamReader(limit=None, loop=None)
+.. class:: StreamReader(limit=_DEFAULT_LIMIT, loop=None)
 
    This class is :ref:`not thread safe <asyncio-multithreading>`.
 
+   The *limit* argument's default value is set to _DEFAULT_LIMIT which is 2**16 (64 KiB)
+
    .. method:: exception()
 
       Get the exception.