[backport#16383 2/3] rpcwallet: document include_watchonly default for watchonly wallets

Summary:
Signed-off-by: William Casarin <jb55@jb55.com>

https://github.com/bitcoin/bitcoin/pull/16383/commits/003a3c73c0450aa18ac2ab2ca47def2b8c53a7df

Depends on D7121

Partial backport of Core PR16383

Test Plan:

ninja

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7122