[bacpkort#10677] RPC Docs: addmultisigaddress is intended for non-watchonly addresses

Summary:
73041c3c99 RPC Docs: addmultisigaddress is intended for non-watchonly addresses (Gregory Sanders)

Pull request description:

Spent a couple hours debugging why my p2sh watchonly funds were not appearing in various accounting calls when address was imported via `addmultisigaddress`.

Backport of Core PR10677

Test Plan:

ninja

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D8180