Document gas heavy functions

[oed]

replaceDelegates has a few loops including garbageCollect
(in RecoveryQuorum.sol#L47)

Since it's behavior has a wide range of gas consumption, the higher end usage should be documented. There should also be explicitly tests for "worst case" scenarios. A goal is to minimize the risk of the contract locking itself up, and understand when low block gas limits would start endangering the contract to be able to evaluate the severity and likelihood of such scenarios, and how large the margin of safety is.