Update documentation to describe None in ast.Interpolation

Author: George-Ogden

Doc/library/ast.rst

@@ -363,6 +363,8 @@ Literals
      function call).
      This has the same meaning as ``FormattedValue.value``.
    * ``str`` is a constant containing the text of the interpolation expression.
+
+     If ``str`` is set to ``None``, then ``value`` is used to generate code when calling :func:`ast.unparse`. This no longer guarantees that the generated code is identical to the original and is intended for code generation.
    * ``conversion`` is an integer:
 
      * -1: no conversion