Added value format description for VOLUME instruction in builder doc

Hi,

This is a simple pull request to improve the documentation.

In the documentation was not mentioned explicitly that VOLUME value
shoud be a valid JSON array. Because of this I spent time to discovering
the problem with my image where I put `VOLUME ['/data']` (with single quotes).
The `['/data']` mount point was parsed and mounted whole as a string without
any errors and warnings.

( In accordance with [current implementation](https://github.com/dotcloud/docker/blob/master/server/buildfile.go#L359-L362) )

Thanks,
Max