text: Title: Incomplete example about query variables
  Body: In the docs, there's an example about how to feed variables to a query [(link)](https://docs.graphene-python.org/en/latest/execution/execute/)
  
  However, the example is incomplete as there is nothing demonstrating how to consume the variables in the resolver:
  
      class Query(graphene.ObjectType):
          user = graphene.Field(User)
  
          def resolve_user(self, info):
              return info.context.get('user')
  
      schema = graphene.Schema(Query)
      result = schema.execute(
          '''query getUser($id: ID) {
              user(id: $id) {
                  id
                  firstName
                  lastName
              }
          }''',
          variables={'id': 12},
      )