doc: Remove mention of renamed mapBlocksUnlinked

Summary:

This has been renamed to m_blocks_unlinked. Instead of adjusting the internal variable name in the help text, explain the debug flag with more general terms.

Backport of Core PR17001

Test Plan: ninja && src/bitcoind -help-debug

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7978