update async docs

src/flask/app.py

@@ -1520,12 +1520,13 @@ class Flask(Scaffold):
         return False
 
     def ensure_sync(self, func):
-        """Ensure that the returned function is sync and calls the async func.
+        """Ensure that the function is synchronous for WSGI workers.
+        Plain ``def`` functions are returned as-is. ``async def``
+        functions are wrapped to run and wait for the response.
+
+        Override this method to change how the app runs async views.
 
         .. versionadded:: 2.0
-
-        Override if you wish to change how asynchronous functions are
-        run.
         """
         if iscoroutinefunction(func):
             return run_async(func)

src/flask/blueprints.py

@@ -475,8 +475,8 @@ class Blueprint(Scaffold):
         """Ensure the function is synchronous.
 
         Override if you would like custom async to sync behaviour in
-        this blueprint. Otherwise :meth:`~flask.Flask..ensure_sync` is
-        used.
+        this blueprint. Otherwise the app's
+        :meth:`~flask.Flask.ensure_sync` is used.
 
         .. versionadded:: 2.0
         """

src/flask/helpers.py

@@ -742,9 +742,10 @@ def run_async(func):
             "Install Flask with the 'async' extra in order to use async views."
         )
 
+    # Check that Werkzeug isn't using its fallback ContextVar class.
     if ContextVar.__module__ == "werkzeug.local":
         raise RuntimeError(
-            "async cannot be used with this combination of Python & Greenlet versions"
+            "Async cannot be used with this combination of Python & Greenlet versions."
         )
 
     @wraps(func)
@@ -763,9 +764,9 @@ def run_async(func):
         async def inner(*a, **k):
             """This restores the context before awaiting the func.
 
-            This is required as the func must be awaited within the
-            context. Simply calling func (as per the
-            copy_current_xxx_context functions) doesn't work as the
+            This is required as the function must be awaited within the
+            context. Only calling ``func`` (as per the
+            ``copy_current_xxx_context`` functions) doesn't work as the
             with block will close before the coroutine is awaited.
             """
             if ctx is not None: