remove ContextVar compat

docs/async-await.rst

@@ -7,8 +7,7 @@ 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]``). It requires Python 3.7+
-where ``contextvars.ContextVar`` is available. This allows views to be
+``async`` extra (``pip install flask[async]``). This allows views to be
 defined with ``async def`` and use ``await``.
 
 .. code-block:: python
@@ -30,6 +29,12 @@ well as all the HTTP method handlers in views that inherit from the
     something like ``ValueError: set_wakeup_fd only works in main thread``,
     please upgrade to Python 3.9.
 
+.. admonition:: Using ``async`` with greenlet
+
+    When using gevent or eventlet to serve an application or patch the
+    runtime, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
+    required.
+
 
 Performance
 -----------

docs/installation.rst

@@ -49,6 +49,18 @@ use them if you install them.
 .. _watchdog: https://pythonhosted.org/watchdog/
 
 
+greenlet
+~~~~~~~~
+
+You may choose to use gevent or eventlet with your application. In this
+case, greenlet>=1.0 is required. When using PyPy, PyPy>=7.3.7 is
+required.
+
+These are not minimum supported versions, they only indicate the first
+versions that added necessary features. You should use the latest
+versions of each.
+
+
 Virtual environments
 --------------------