Comments on clearing last collect

[DanielVF]

During the audit we made the following two changes:

Removed from FixedRateRewardsSource.sol initialize:

 _setRewardsPerSecond(_rewardsPerSecond); // removed

Added to FixedRateRewardsSource.sol _setRewardsPerSecond:

if (_config.rewardsPerSecond == 0) {
   _config.lastCollect = uint64(block.timestamp);
}

These make it so that when rewards are enabled on the contract, the lastCollect is reset to the current time, and thus no rewards will be paid out for the period when the rewards were set to zero.

In other case, as before, the contract will pay out pending rewards from the lastCollect at the newly set rate. Calling collectRewards before setting the rate allows precise reward payments for past rewards.

This PR just updates the comments on this change.