Add docs on provider async mode enabling

docs/providers/async.rst

@@ -15,3 +15,20 @@ Providers support asynchronous injections.
    :language: python
    :emphasize-lines: 26-29
    :lines: 3-
+
+If provider has any awaitable injections it turns into async mode. In async mode provider always returns awaitable.
+This causes a cascade effect:
+
+.. code-block:: bash
+
+   provider1()              <── Async mode enabled <──┐
+   │                                                  │
+   ├──> provider2()                                   │
+   │                                                  │
+   ├──> provider3()         <── Async mode enabled <──┤
+   │    │                                             │
+   │    └──> provider4()    <── Async provider ───────┘
+   │
+   └──> provider5()
+        │
+        └──> provider6()