Update PEP-572 for f-strings (see bpo-36798) (#1024)

See bpo-36798.

This update specifies the interplay between f-strings and
the assignment operator. Specifically, it attempts to
clarify how to use assignment operators inside of f-strings.

pep-0572.rst

@@ -208,6 +208,20 @@ in order to avoid ambiguities or user confusion:
   used more than once, the expression is likely to need parenthesizing anyway,
   so this prohibition will rarely affect code.
 
+- Assignment expressions inside of f-strings require parentheses. Example::
+
+    >>> f'{(x:=10)}'  # Valid, uses assignment expression
+    '10'
+    >>> x = 10
+    >>> f'{x:=10}'    # Valid, passes '=10' to formatter
+    '        10'
+
+  This shows that what looks like an assignment operator in an f-string is
+  not always an assignment operator.  The f-string parser uses ``:`` to
+  indicate formatting options.  To preserve backwards compatibility,
+  assignment operator usage inside of f-strings must be parenthesized.
+  As noted above, this usage of the assignment operator is not recommended.
+
 Scope of the target
 -------------------