Restore customization of the Couchbase cache manager

With the upgrade to the new Couchbase SDK and the related changes in
Spring Data Couchbase, CacheManagerCustomizer can no longer be used to
customize the Couchbase cache manager as it is an immutable class.

This commit introduces a dedicated callback for the
CouchbaseCacheManagerBuilder that is used by the auto-configuration and
update the documentation to refer to it with a sample usage.

Closes gh-22573

spring-boot-project/spring-boot-docs/src/main/java/org/springframework/boot/docs/cache/CouchbaseCacheManagerCustomizationExample.java

@@ -0,0 +1,47 @@
+/*
+ * Copyright 2012-2020 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.boot.docs.cache;
+
+import java.time.Duration;
+
+import org.springframework.boot.autoconfigure.cache.CouchbaseCacheManagerBuilderCustomizer;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.data.couchbase.cache.CouchbaseCacheConfiguration;
+
+/**
+ * An example how to customize {@code CouchbaseCacheManagerBuilder} via
+ * {@code CouchbaseCacheManagerBuilderCustomizer}.
+ *
+ * @author Dmytro Nosan
+ */
+@Configuration(proxyBeanMethods = false)
+public class CouchbaseCacheManagerCustomizationExample {
+
+	// tag::configuration[]
+	@Bean
+	public CouchbaseCacheManagerBuilderCustomizer myCouchbaseCacheManagerBuilderCustomizer() {
+		return (builder) -> builder
+				.withCacheConfiguration("cache1",
+						CouchbaseCacheConfiguration.defaultCacheConfig().entryExpiry(Duration.ofSeconds(10)))
+				.withCacheConfiguration("cache2",
+						CouchbaseCacheConfiguration.defaultCacheConfig().entryExpiry(Duration.ofMinutes(1)));
+
+	}
+	// end::configuration[]
+
+}