bpo-39545: Document changes in the support of await in f-strings. (GH-18456)

https://bugs.python.org/issue39545
(cherry picked from commit f632736)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Author: miss-islington

Doc/reference/lexical_analysis.rst

@@ -685,6 +685,11 @@ strings), but they cannot contain comments.  Each expression is evaluated
 in the context where the formatted string literal appears, in order from
 left to right.
 
+.. versionchanged:: 3.7
+   Prior to Python 3.7, an :keyword:`await` expression and comprehensions
+   containing an :keyword:`async for` clause were illegal in the expressions
+   in formatted string literals due to a problem with the implementation.
+
 If a conversion is specified, the result of evaluating the expression
 is converted before formatting.  Conversion ``'!s'`` calls :func:`str` on
 the result, ``'!r'`` calls :func:`repr`, and ``'!a'`` calls :func:`ascii`.

Doc/whatsnew/3.7.rst

@@ -493,6 +493,11 @@ of this mode.
 Other Language Changes
 ======================
 
+* An :keyword:`await` expression and comprehensions containing an
+  :keyword:`async for` clause were illegal in the expressions in
+  :ref:`formatted string literals <f-strings>` due to a problem with the
+  implementation.  In Python 3.7 this restriction was lifted.
+
 * More than 255 arguments can now be passed to a function, and a function can
   now have more than 255 parameters. (Contributed by Serhiy Storchaka in
   :issue:`12844` and :issue:`18896`.)