doc: Add documentation for 'checklevel' argument in 'verifychain' RPC call

Summary:

Rationale: When bitcoin-cli help verifychain is called, the user doesn't get any documentation about the checklevel argument, leading to issues like #18995.

This PR addresses that issue and adds documentation for what each level does, and that each level includes the checks of the previous levels.

This is a backport of core#19005

Test Plan:
src/bitcoind -help-debug | less

src/bitcoin-cli help checklevel

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9278