document split brain scenario and the system behaviour

[raffaelespazzoli]

in order to gain confidence on this operator, there needs to be documentation on what the expected behavior is during network partitioning situations (a.k.a. split brain scenario).
Confidence in particularly importation for this operator as it is going to be in many cases the linchpin of the DR strategy.

[ytsarev]

Docs on existing split brain mechanisms are missing for good. We need to add them.

We also have #175 for a while to deterministically test the possible degraded behavior.

[ytsarev]

@tanujd11 kindly agreed to share his latest experience in split brain