* Initial draft of multitenancy doc

* Split into semantic lines for better diffing, some tweaks

* Wording tweak

Co-authored-by: Dave Fellows <[email protected]>

* Clarify that flexible credential matching is a future plan

Since it's orthogonal to the changes to allow multiple operators per
cluster.

* Wording tweaks

Co-authored-by: Dave Fellows <[email protected]>
Co-authored-by: Matthew Christopher <[email protected]>