A comment in https://github.com/docker/docker/issues/6354#issuecomment-74160215
brought to light that the "Managing Data in containers" section contained an
incorrect (or confusing) line;
"Volumes persist until no containers use them"
Which implies that volumes are automatically removed if they are no longer
referenced by a container.
This pull-request attempts to add some information explaining that volumes are
never automatically removed by Docker and adds some extra hints on working
with data volumes.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>