[backport#15870 3/3][doc] rpcwallet: Only fail rescan when blocks have been pruned

Summary:
https://github.com/bitcoin/bitcoin/pull/15870/commits/fa7e311e169349bfcf1dab8b980724e8ddf4e749

Depends on D7015

Concludes backport of Core PR15870

Test Plan:

none

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7016