doc: Add internal interface conventions to developer notes

Summary:
Backport of Core PR18278 part [6/6] : https://github.com/bitcoin/bitcoin/pull/18278/commits/3dc27a15242a22b5301904375e5880372e9b7f4d

Depends on D8023

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8024