[backport#15831] test: Add test that addmultisigaddress fails for watchonly addresses

Summary:
test: Fixup creatmultisig documentation and whitespace

Backport of Core PR15831

Test Plan:

test_runner.py rpc_createmultisig

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6550