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

Summary:
Backport of Core [[https://github.com/bitcoin/bitcoin/pull/15223 | 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

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