validationinterface: Rework documentation, Rename pwalletIn to callbacks

Summary:
This is a backport of Core PR18742 [1/3]
https://github.com/bitcoin/bitcoin/pull/18742/commits/fa770ce7fe67685c43780e219d8232efbee0bb8e

Test Plan: ninja

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9103