With the completion of MONGOID-4237, we will have added a new global flag which will allow the user to specify that they want BigDecimals to be stored as Decimal128s in the database instead of strings. Document this, and explain the drawbacks of using Decimal128 instead of strings:

1. Decimal128 has range and precision limitations 
2. Decimal128 has signed NaNs
3. Decimal128 keeps track of trailing zeros