doc: add note on precedence of options in bitcoin.conf

Summary:
Backport of Core PR16448

Depends on D7890

Test Plan: Proof-reading 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/D7891