[docs] use consistent naming for possible_overwrite

Summary:
And other general comment improvements for adding coins.

This is a backport of Core PR18410 [3/3]
https://github.com/bitcoin/bitcoin/pull/18410/commits/21fa0a44abe8c1b5c452e097eab20cf0ae988805
Depends on D8954

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8955