bpo-13814: Explain why generators are not context managers (GH-26835)

Put entry in Design FAQ after a question about a context manager for assignment.
Original patch by Aidan Lowe.
(cherry picked from commit 51f45d08)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Doc/faq/design.rst

@@ -709,6 +709,15 @@ bindings are resolved at run-time in Python, and the second version only needs
 to perform the resolution once.
 
 
+Why don't generators support the with statement?
+------------------------------------------------
+
+For technical reasons, a generator used directly as a context manager
+would not work correctly.  When, as is most common, a generator is used as
+an iterator run to completion, no closing is needed.  When it is, wrap
+it as "contextlib.closing(generator)" in the 'with' statment.
+
+
 Why are colons required for the if/while/def/class statements?
 --------------------------------------------------------------
 

Misc/NEWS.d/next/Documentation/2021-06-21-15-46-32.bpo-13814.LDcslu.rst

+In the Design FAQ, answer "Why don't generators support the with statement?"