pythongh-86482: Document assignment expression need for ()s (python#23291)

Co-authored-by: Jelle Zijlstra <[email protected]>

Author: terryjreedy

Doc/reference/expressions.rst

@@ -1766,6 +1766,13 @@ Or, when processing a file stream in chunks:
    while chunk := file.read(9000):
        process(chunk)
 
+Assignment expressions must be surrounded by parentheses when used
+as sub-expressions in slicing, conditional, lambda,
+keyword-argument, and comprehension-if expressions
+and in ``assert`` and ``with`` statements.
+In all other places where they can be used, parentheses are not required,
+including in ``if`` and ``while`` statements.
+
 .. versionadded:: 3.8
    See :pep:`572` for more details about assignment expressions.
 

Misc/NEWS.d/next/Core and Builtins/2020-11-15-02-08-43.bpo-42316.LqdkWK.rst

@@ -0,0 +1 @@
+Document some places where an assignment expression needs parentheses.