Merge #14206: doc: Document -checklevel levels

Summary:
8e1c13e6da2a5e1054503539664c139f1eb91e90 doc: Document -checklevel levels (Wladimir J. van der Laan)

Pull request description:

Document the various possible check levels for the command-line argument. The numbers say nothing on their own.

Tree-SHA512: 8a526c53222b55304dde1d9350dd15a50f1dd62bf452a32dc886a4521e3ab49d5f0a86a4c5cbb0d52fb76b60222101045100f93fee5e1a5d5e3ab8e79b64cbe0

Backport of Core PR14206
https://github.com/bitcoin/bitcoin/pull/14206/

Complete T581

Test Plan:

make check
./bitcoind --help -help-debug

Verify help text change

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: Fabien, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4737