Update to MongoDb specification regarding coefficient encoding in BSON decimal

Description

At the repo: 

https://github.com/mongodb/specifications

At page:

https://github.com/mongodb/specifications/blob/master/source/bson-decimal128/decimal128.rst

The specification declares that the decimal function for MongoDB implements the http://speleotrove.com/decimal/decbits.html specification. However, that specification explicitly declares that the coefficient is encoded using Densely Packed Decimal (DPD) while the actual drivers encode using Binary Integer Decimal. This has caused at least two driver authors to originally write the wrong code. The error is easily addressed with a few minor changes in text.

I have a pending PR for this: https://github.com/mongodb/specifications/pull/795. Further details are in the PR.

I have notice that PRs with a declared issue in Jira get reviewed and possibly merged, so I'm adding this issue to help this out.

Scope of changes

Impact to Other Docs

MVP (Work and Date)

Resources (Scope or Design Docs, Invision, etc.)