Planner rolling updates doc clarification (#863)

* Add some clarification in the DisaggSet docs for how rolling updates work across roles and a comment in the planner

Signed-off-by: BenjaminBraunDev <benjaminbraun@google.com>

* Update example rolling update table to be more detailed

* Adjust text formatting

* Promote example to a header

* add example to TOC

* Update rolling-update docs with initialOld terminology

---------

Signed-off-by: BenjaminBraunDev <benjaminbraun@google.com>