Update documentation for @typePolicy

kotlin_labs/v0.5/kotlin_labs-v0.5.graphql

@@ -12,7 +12,42 @@ Since: 3.0.0
 """
 directive @typePolicy(
     """
-    a selection set containing fields used to compute the cache key of an object. Order is important.
+    A selection set containing fields used to compute the cache key of an object.
+    Nested selection sets are currently not supported. Order is important.
+
+    Key fields can be defined on interfaces. In that case, the key fields apply to all sub-types and sub-types are not allowed to define their own key fields.
+    If a type implements multiple interfaces with keyfields, the keyfields must match across all interfaces with keyfields.
+
+    The key fields are automatically added to the operations by the compiler.
+    Aliased key fields are not recognized and the compiler adds a non-aliased version of the field if that happens.
+    If a type is queried through an interface/union, this may add fragments.
+
+    For an example, this query:
+
+    ```graphql
+    query {
+        product {
+            price
+        }
+    }
+    ```
+
+    is turned into this one after compilation:
+
+    ```graphql
+    query {
+        product {
+            ... on Book {
+                isbn
+            }
+            ... on Movie {
+                id
+            }
+            price
+        }
+    }
+    ```
+
     """
     keyFields: String! = "",
     """