Improve @Cacheable documentation regarding java.util.Optional

This commit improves the documentation for @Cacheable to point out that
`null` will be stored in the cache for an empty `Optional` return value.

Closes gh-27184

spring-context/src/main/java/org/springframework/cache/annotation/Cacheable.java

 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -38,9 +38,11 @@ import org.springframework.core.annotation.AliasFor;
  * replace the default one (see {@link #keyGenerator}).
  *
  * <p>If no value is found in the cache for the computed key, the target method
- * will be invoked and the returned value stored in the associated cache. Note
- * that Java8's {@code Optional} return types are automatically handled and its
- * content is stored in the cache if present.
+ * will be invoked and the returned value will be stored in the associated cache.
+ * Note that {@link java.util.Optional} return types are unwrapped automatically.
+ * If an {@code Optional} value is {@linkplain java.util.Optional#isPresent()
+ * present}, it will be stored in the associated cache. If an {@code Optional}
+ * value is not present, {@code null} will be stored in the associated cache.
  *
  * <p>This annotation may be used as a <em>meta-annotation</em> to create custom
  * <em>composed annotations</em> with attribute overrides.

src/docs/asciidoc/integration.adoc

@@ -5688,7 +5688,7 @@ method -- if at least one cache is hit, the associated value is returned.
 NOTE: All the other caches that do not contain the value are also updated, even though
 the cached method was not actually invoked.
 
-The following example uses `@Cacheable` on the `findBook` method:
+The following example uses `@Cacheable` on the `findBook` method with multiple caches:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
@@ -5896,9 +5896,11 @@ want to cache paperback books, as the following example does:
 <1> Using the `unless` attribute to block hardbacks.
 
 
-The cache abstraction supports `java.util.Optional`, using its content as the cached value
-only if it is present. `#result` always refers to the business entity and never a
-supported wrapper, so the previous example can be rewritten as follows:
+The cache abstraction supports `java.util.Optional` return types. If an `Optional` value
+is _present_, it will be stored in the associated cache. If an `Optional` value is not
+present, `null` will be stored in the associated cache. `#result` always refers to the
+business entity and never a supported wrapper, so the previous example can be rewritten
+as follows:
 
 [source,java,indent=0,subs="verbatim,quotes"]
 ----
@@ -5906,8 +5908,8 @@ supported wrapper, so the previous example can be rewritten as follows:
 	public Optional<Book> findBook(String name)
 ----
 
-Note that `result` still refers to `Book` and not `Optional`. As it might be `null`, we
-should use the safe navigation operator.
+Note that `#result` still refers to `Book` and not `Optional<Book>`. Since it might be
+`null`, we use SpEL's <<core.adoc#expressions-operator-safe-navigation, safe navigation operator>>.
 
 [[cache-spel-context]]
 ===== Available Caching SpEL Evaluation Context