Add release notes for importmulti descriptor support

Summary:
This is a partial backport of Core PR14491 : https://github.com/bitcoin/bitcoin/pull/14491/commits/b985e9c850ea682eced7021faf6c7c835066c61b

Depends on D6192

Test Plan: read

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6193