Improve Javadoc of ContentCachingRequestWrapper

Closes gh-27068

spring-web/src/main/java/org/springframework/web/util/ContentCachingRequestWrapper.java

 /*
- * Copyright 2002-2018 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.
@@ -40,9 +40,15 @@ import org.springframework.lang.Nullable;
  * the {@linkplain #getInputStream() input stream} and {@linkplain #getReader() reader},
  * and allows this content to be retrieved via a {@link #getContentAsByteArray() byte array}.
  *
+ * <p>This class acts as an interceptor that only caches content as it is being
+ * read but otherwise does not cause content to be read. That means if the request
+ * content is not consumed, then the content is not cached, and cannot be
+ * retrieved via {@link #getContentAsByteArray()}.
+ *
  * <p>Used e.g. by {@link org.springframework.web.filter.AbstractRequestLoggingFilter}.
  * Note: As of Spring Framework 5.0, this wrapper is built on the Servlet 3.1 API.
  *
+ *
  * @author Juergen Hoeller
  * @author Brian Clozel
  * @since 4.1.3
@@ -184,6 +190,10 @@ public class ContentCachingRequestWrapper extends HttpServletRequestWrapper {
 	/**
 	 * Return the cached request content as a byte array.
 	 * <p>The returned array will never be larger than the content cache limit.
+	 * <p><strong>Note:</strong> The byte array returned from this method
+	 * reflects the amount of content that has has been read at the time when it
+	 * is called. If the application does not read the content, this method
+	 * returns an empty array.
 	 * @see #ContentCachingRequestWrapper(HttpServletRequest, int)
 	 */
 	public byte[] getContentAsByteArray() {