Clarify difference between statuses in incident response plan

[AetherUnbound]

Description

The current set of statuses for an incident leave some confusion around which status is appropriate to move to after Stabilization pending.

Quoted from the discussion here:

@sarayoufriend:
What's the difference between stabilized and under investigation? Is that to allow specific time after stabilisation where we haven't started looking into the root cause?

@stacimc:
I also had this question. Per the new text added below:

Some incidents may not require further investigation once they are stabilized and can be immediately resolved.

But I think those incidents would go Stabilization pending straight to Resolved, so I'm still not sure when something would be Stabilized but not Under investigation. I guess it implies that investigation is needed but not started? Or, possibly, investigation has already happened and the cause is identified but not yet fixed? Although that can't be right, because that's the definition of Resolved given below.

If Resolved is updated to mean that the long-term fixes are implemented, then one interpretation of the statuses which would maybe make sense is:

• Stabilization pending: actively disrupting service
• Under investigation: stabilized but not resolved. Root causes are being researched.
• Stabilized: Root causes identified but not yet fixed
• Resolved: Long-term fixes applied

But then the name stabilized feels like an inadequate description 😓

Additional context

Came from discussion around the initial incident response plan in #2679