update docs on context providers

docs/providers/context-providers.md

@@ -21,13 +21,13 @@ class StorageServiceLocal(StorageService):
 class StorageServiceRemote(StorageService):
     ...
 
-def context_adapter_function(*, storage_backend: str | None = None, **_: object) -> str:
+def selector_function(*, storage_backend: str | None = None, **_: object) -> str:
     return storage_backend or "local"
 
 class Dependencies(BaseGraph):
     storage_service: providers.Selector[StorageService] = providers.Selector(
         Scope.APP,
-        lambda: os.getenv("STORAGE_BACKEND", "local"),
+        selector_function,
         local=providers.Factory(Scope.APP, StorageServiceLocal),
         remote=providers.Factory(Scope.APP, StorageServiceRemote),
     )
@@ -36,3 +36,22 @@ with Container(scope=Scope.APP, context={"storage_backend": "remote"}) as contai
     print(type(Dependencies.storage_service.sync_resolve(container)))
     # StorageServiceRemote
 ```
+
+## ContextAdapter
+- Receives context unpacked to callable object.
+- Can be used in another providers to access data from context.
+
+```python
+from modern_di import BaseGraph, Container, Scope, providers
+
+
+def context_adapter_function(*, storage_backend: str | None = None, **_: object) -> str:
+    return storage_backend or "local"
+
+class Dependencies(BaseGraph):
+    context_adapter = providers.ContextAdapter(Scope.APP, context_adapter_function)
+
+with Container(scope=Scope.APP, context={"storage_backend": "remote"}) as container:
+    print(Dependencies.context_adapter.sync_resolve(container))
+    # "remote"
+```