Doc: add information about security to the JSON-RPC doc

Summary:
Backport of Core PR15223
Depends on D7765

Test Plan:
Replaced mentions of "Core" by "ABC".
Visual inspection in a markdown viewer.

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7766