Polish BeanRegistrar Javadoc and add tests for non-invocation semantics

Revise the BeanRegistrar Javadoc to document the two distinct usage
modes: @Configuration/@Import and programmatic GenericApplicationContext
setup.

Clarify that implementations are not Spring components (requiring a
no-arg constructor and no dependency injection), and detail the ordering
guarantees for each mode.

Add missing tests

Signed-off-by: Stéphane Nicoll <stephane.nicoll@broadcom.com>

spring-context/src/main/java/org/springframework/context/DeferredBeanRegistrar.java

@@ -1,46 +0,0 @@
-/*
- * Copyright 2002-present the original author or authors.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *      https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-package org.springframework.context;
-
-import org.springframework.beans.factory.BeanRegistrar;
-import org.springframework.beans.factory.BeanRegistry;
-import org.springframework.core.ParameterizedTypeReference;
-import org.springframework.core.env.Environment;
-
-/**
- * A variant of {@link BeanRegistrar} which aims to be invoked
- * at the end of the bean registration phase, coming after regular
- * bean definition reading and configuration class processing.
- *
- * <p>This allows for seeing all user-registered beans, potentially
- * reacting to their presence. The {@code containsBean} methods on
- * {@link BeanRegistry} will provide reliable answers, independent
- * of the order of user bean registration versus {@code BeanRegistrar}
- * import/registration.
- *
- * @author Juergen Hoeller
- * @since 7.1
- * @see #register(BeanRegistry, Environment)
- * @see BeanRegistry#containsBean(String)
- * @see BeanRegistry#containsBean(Class)
- * @see BeanRegistry#containsBean(ParameterizedTypeReference)
- * @see org.springframework.context.support.GenericApplicationContext#register( BeanRegistrar...)
- * @see org.springframework.context.annotation.Import
- */
-public interface DeferredBeanRegistrar extends BeanRegistrar {
-
-}