Fix self-consistency issues in proposal repo

[ptomato]

The README describes a maximumSignificantDigits option to Decimal.round() while the Decimal128 and BigDecimal references do not.

There are probably more of these discrepancies. We should do a run-through of the proposal and decide which one is meant in each case, and then make sure that there is only one place where these things are described so they can no longer get out of sync.