Update Javadoc of InputStreamResource#contentLength

Closes gh-20990

Author: rstoyanchev

spring-core/src/main/java/org/springframework/core/io/AbstractResource.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -150,9 +150,11 @@ public ReadableByteChannel readableChannel() throws IOException {
 	}
 
 	/**
-	 * This implementation reads the entire InputStream to calculate the
-	 * content length. Subclasses will almost always be able to provide
-	 * a more optimal version of this, e.g. checking a File length.
+	 * This method reads the entire InputStream to determine the content length.
+	 * <p>For a custom sub-class of {@code InputStreamResource}, we strongly
+	 * recommend overriding this method with a more optimal implementation, e.g.
+	 * checking File length, or possibly simply returning -1 if the stream can
+	 * only be read once.
 	 * @see #getInputStream()
 	 */
 	@Override