Improve description for @OneOf directive

Replicates graphql/graphql-js@acf05e3

Author: Cito

src/graphql/type/directives.py

@@ -261,12 +261,13 @@ def assert_directive(directive: Any) -> GraphQLDirective:
     description="Exposes a URL that specifies the behavior of this scalar.",
 )
 
-# Used to declare an Input Object as a OneOf Input Objects.
+# Used to indicate an Input Object is a OneOf Input Object.
 GraphQLOneOfDirective = GraphQLDirective(
     name="oneOf",
     locations=[DirectiveLocation.INPUT_OBJECT],
     args={},
-    description="Indicates an Input Object is a OneOf Input Object.",
+    description="Indicates exactly one field must be supplied"
+    " and this field must not be `null`.",
 )
 
 specified_directives: tuple[GraphQLDirective, ...] = (

tests/utilities/test_print_schema.py

@@ -771,7 +771,9 @@ def prints_introspection_schema():
               url: String!
             ) on SCALAR
 
-            """Indicates an Input Object is a OneOf Input Object."""
+            """
+            Indicates exactly one field must be supplied and this field must not be `null`.
+            """
             directive @oneOf on INPUT_OBJECT
 
             """