Expected max error for FixedPointMathLib.rpow

[beeb]

Hey there! I found some values for which rpow has a pretty large error, and I was wondering if that's expected? Unlike powWad, this function does not document that it's imprecise.

For example:

// res = 990049833743622642
uint256 res = FixedPointMathLib.rpow(999999999999999999, 1e16, 1e18);

The error is about 5545412 compared to the true value 990049833749168054 (rounded to nearest).

If that's unexpected, I can open an issue.