Closes #9296 adds a more detailed note about the mount behavior with -v flag

Signed-off-by: Andrew C. Bodine <acbodine@us.ibm.com>

docs/sources/userguide/dockervolumes.md

@@ -51,8 +51,15 @@ directory from your own host into a container.
 
     $ sudo docker run -d -P --name web -v /src/webapp:/opt/webapp training/webapp python app.py
 
-This will mount the local directory, `/src/webapp`, into the container as the
-`/opt/webapp` directory. This is very useful for testing, for example we can
+This will mount the host directory, `/src/webapp`, into the container at
+`/opt/webapp`.
+
+> **Note:**
+> If the path `/opt/webapp` already exists inside the container's image, it's
+> contents will be replaced by the contents of `/src/webapp` on the host to stay
+> consistent with the expected behavior of `mount`
+
+This is very useful for testing, for example we can
 mount our source code inside the container and see our application at work as
 we change the source code. The directory on the host must be specified as an
 absolute path and if the directory doesn't exist Docker will automatically