bpo-23952: Document cgi module's maxlen variable

Author: hugovk

Doc/library/cgi.rst

@@ -21,6 +21,11 @@ Support module for Common Gateway Interface (CGI) scripts.
 This module defines a number of utilities for use by CGI scripts written in
 Python.
 
+The ``maxlen`` variable can be set to an integer indicating the maximum size
+of a POST request. POST requests larger than this size will result in a
+:exc:`ValueError` being raised during parsing. The default value of this
+variable is ``0``, meaning the request size is unlimited.
+
 
 Introduction
 ------------