wallet: Fixup rescanblockchain result doc

Summary:
Backport of core PR15279.
This is only the remaining as most has been ported already (see D5953)

Test Plan:

bitcoin-cli help rescanblockchain

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D5955