Clarify docs for rsync ownership

I've only ever reached the rsync docs from Google, which I imagine is pretty common. It's not clear that the options for rsync shared folders also include global options applicable to any shared folder. Specifically, it's very confusing when customizing the rsync command, and not having ownership match what the args are setting.

This PR adds a link up to the basic usage docs (which should jump out a little more), as well as indicates that 'owner' and 'group' supersede any rsync options.