Add descriptor reference documentation

Summary:

• Adjust TODO link
• Replace duplcate reference with reference to reference doc

In addition, add release notes.

Depends on D4421

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4422