[backport#16397] doc: Clarify includeWatching for fundrawtransaction

Summary:
Clarify includeWatching for fundrawtransaction (Steven Roose)

Pull request description:

Might be sufficient to solve https://github.com/bitcoin/bitcoin/issues/16396, https://github.com/bitcoin/bitcoin/issues/7879 and https://github.com/bitcoin/bitcoin/issues/14405.

Backport of Core PR16397

Test Plan:

ninja
bitcoin-cli -regtest help fundrawtransaction

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7381