[backport#13756] bitcoind: update -avoidpartialspends description to account for auto-enable for avoid_reuse wallets

Summary:
https://github.com/bitcoin/bitcoin/pull/13756/commits/5ebc6b0eb267e0552c66fffc5e5afe7df8becf80

Depends on D6468

Concludes backport of Core PR13756

Test Plan:

ninja check
./src/bitcoind -h | grep avoid -C 3

check output is correct

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6469