[rpc] Correct reconsiderblock help text, add test

Summary:

Rework documentation and test to match the implementation

Backport of core PR15057.

Test Plan:

./test/functional/test_runner.py rpc_invalidateblock

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7983