Revert 1213 update mutation docs (#1214)

* Revert "Update requirement for Query type in mutation docs (#1213)"

This reverts commit a9625dac0e.

* Add test to check that Query type must be defined

docs/types/mutations.rst

@@ -48,7 +48,11 @@ So, we can finish our schema like this:
     class MyMutations(graphene.ObjectType):
         create_person = CreatePerson.Field()
 
-    schema = graphene.Schema(mutation=MyMutations)
+    # We must define a query for our schema
+    class Query(graphene.ObjectType):
+        person = graphene.Field(Person)
+
+    schema = graphene.Schema(query=Query, mutation=MyMutations)
 
 Executing the Mutation
 ----------------------

graphene/types/schema.py

@@ -381,7 +381,7 @@ class Schema:
     questions about the types through introspection.
 
     Args:
-        query (Optional[Type[ObjectType]]): Root query *ObjectType*. Describes entry point for fields to *read*
+        query (Type[ObjectType]): Root query *ObjectType*. Describes entry point for fields to *read*
             data in your Schema.
         mutation (Optional[Type[ObjectType]]): Root mutation *ObjectType*. Describes entry point for
             fields to *create, update or delete* data in your API.

graphene/types/tests/test_schema.py

@@ -59,3 +59,12 @@ def test_schema_str():
 def test_schema_introspect():
     schema = Schema(Query)
     assert "__schema" in schema.introspect()
+
+
+def test_schema_requires_query_type():
+    schema = Schema()
+    result = schema.execute("query {}")
+
+    assert len(result.errors) == 1
+    error = result.errors[0]
+    assert error.message == "Query root type must be provided."