bpo-33804: Document that the argument for io.TextIOBase.read() is optional (GH-7510)

Doc/library/io.rst

@@ -799,7 +799,7 @@ Text I/O
 
       .. versionadded:: 3.1
 
-   .. method:: read(size)
+   .. method:: read(size=-1)
 
       Read and return at most *size* characters from the stream as a single
       :class:`str`.  If *size* is negative or ``None``, reads until EOF.