bpo-23952: Document cgi module's maxlen variable

Author: hugovk

Lib/cgi.py

@@ -13,6 +13,11 @@
 
 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 ValueError
+being raised during parsing. The default value of this variable is 0, meaning
+the request size is unlimited.
 """
 
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