Update doc about minimal Python version for async support

Author: laggardkernel

docs/async-await.rst

@@ -7,7 +7,8 @@ Using ``async`` and ``await``
 
 Routes, error handlers, before request, after request, and teardown
 functions can all be coroutine functions if Flask is installed with the
-``async`` extra (``pip install flask[async]``). This allows views to be
+``async`` extra (``pip install flask[async]``). It requires Python 3.7+
+where ``contextvars.ContextVar`` is available. This allows views to be
 defined with ``async def`` and use ``await``.
 
 .. code-block:: python

docs/installation.rst

@@ -8,6 +8,8 @@ Python Version
 We recommend using the latest version of Python. Flask supports Python
 3.6 and newer.
 
+``async`` support in Flask requires Python 3.7+ for ``contextvars.ContextVar``.
+
 
 Dependencies
 ------------