Mention new descriptor RPCs in descriptors.md
Mention new descriptor RPCs in descriptors.md

Github-Pull: #15757
Rebased-From: 9b085f4863eaefde4bec0638f1cbc8509d6ee59a

chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin,chaincoin/chaincoin