Merge #11753: clarify abortrescan rpc use

Summary:
8b2c733 clarify abortrescan rpc use (Gregory Sanders)

Pull request description:

`-rescan` during startup doesn't apply since RPC has not warmed up by that point

Tree-SHA512: 0b6738ed9429ca41edbfa23bb3e2da23bb16fed9e55c1803ab140b2efc49ef3295bb8b7eeb8c24df94204c5b3fb25d0d57cbc8d1be11856a5763c9fde71bceba

Backport of Core PR11753
https://github.com/bitcoin/bitcoin/pull/11753/

Test Plan:

make check
./bitcoind
./bitcoin-cli help abortrescan

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3777