Docker: First Merged to master Contribution to docker.github.io

 I have a new project that will use storage in Docker (see Manage data in Docker). I found one section of the documentation to be confusing because it does not distinguish between persisted and in-memory file storage: 

I rewrote the section and it was approved:

And merged into master for docker.github.io:

Here is the original text:

Here is my change: