improve docs for split/aggregated view

[slartibart70]

I'm having trouble understanding the usefulness of 'A' split view in a multi-layered image.
It seems, the newly opened split view only shows the first layer without option to change this?
I'm thinking of visual compare between layers where split would be useful (esp. with the file-view option)

[deveshctl]

It's not actually locked to the first layer — tab over to the layers pane and move up/down, both halves follow the selected layer. Top half is what that layer changed, bottom is the full filesystem up to that point (not a layer-vs-layer diff, which sounds like what you were expecting).

Fair point that none of that is obvious from the UI though — that's on me to document better. Leaving this open for the docs fix.