doc: Clarify nPruneAfterHeight

Summary:
This is a partial backport of core#23974
https://github.com/bitcoin/bitcoin/pull/23974/commits/faa8c2d7d75f8d9782709e73e00e35851e233392

Depends on D12513

Test Plan: proofreading

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12514