Document that CacheManager.getCache() may create a new Cache

Prior to this commit, it was unclear that an invocation of getCache()
might potentially create a new Cache.

This commit updates the Javadoc to point out that concrete CacheManager
implementations may choose to create a new Cache at runtime if the Cache
does not already exist.

Closes gh-23193

spring-context/src/main/java/org/springframework/cache/CacheManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -22,23 +22,28 @@ import org.springframework.lang.Nullable;
 
 /**
  * Spring's central cache manager SPI.
- * Allows for retrieving named {@link Cache} regions.
+ *
+ * <p>Allows for retrieving named {@link Cache} regions.
  *
  * @author Costin Leau
+ * @author Sam Brannen
  * @since 3.1
  */
 public interface CacheManager {
 
 	/**
-	 * Return the cache associated with the given name.
+	 * Get the cache associated with the given name.
+	 * <p>Note that the cache may be created at runtime if the native provider
+	 * supports it.
 	 * @param name the cache identifier (must not be {@code null})
-	 * @return the associated cache, or {@code null} if none found
+	 * @return the associated cache, or {@code null} if such a cache does not
+	 * exist or could be not created
 	 */
 	@Nullable
 	Cache getCache(String name);
 
 	/**
-	 * Return a collection of the cache names known by this manager.
+	 * Get a collection of the cache names known by this manager.
 	 * @return the names of all caches known by the cache manager
 	 */
 	Collection<String> getCacheNames();