Define "cap device"

[jbaublitz]

We don't have a clear definition in our documentation about what the cap device is. I'm thinking something along the lines of:
"The cap device is the device on top that unifies the address space of all devices in the pool. In the case of caching, this is the cap device. In the case of no caching, this is a linear device across all devices in the pool."

@mulkieran @bmr-cymru Thoughts?

[bmr-cymru]

I think it's worth adding something along those lines - the doc is quite careful to define other terms that it uses, and it left me scratching my head a bit and wondering if I'd missed something.