add documentation clarifying behavior of VOLUME instruction

Signed-off-by: Patrick Hemmer <patrick.hemmer@gmail.com>

docs/reference/builder.md

@@ -912,6 +912,10 @@ This Dockerfile results in an image that causes `docker run`, to
 create a new mount point at `/myvol` and copy the  `greeting` file 
 into the newly created volume.
 
+> **Note**:
+> If any build steps change the data within the volume after it has been
+> declared, those changes will be discarded.
+
 > **Note**:
 > The list is parsed as a JSON array, which means that
 > you must use double-quotes (") around words not single-quotes (').