gh-86482: Document assignment expression need for ()s

Doc/reference/expressions.rst

@@ -1745,6 +1745,11 @@ 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 subscript, slicing, conditional, lambda,
+keyword-argument, generator, and comprehension-if expressions.  They
+can only be used as is in if and while statements and in decorators.
+
 .. 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.