doc: Tidy up nMinDiskSpace comment

Summary:
nMinDiskSpace was removed in commit rABCd181505a6b0f

Also, remove incorrect doxygen comment.
See https://doxygen.bitcoincore.org/class_c_chain.html#aeb563751f7362d4308c7c2cb35b834a5

This is a backport of core#23600

Test Plan: Proof-reading and grep -r nMinDiskSpace src/.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12415