doc: Document -checklevel levels

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

src/init.cpp

@@ -426,7 +426,14 @@ void SetupServerArgs()
 #endif
 
     gArgs.AddArg("-checkblocks=<n>", strprintf("How many blocks to check at startup (default: %u, 0 = all)", DEFAULT_CHECKBLOCKS), true, OptionsCategory::DEBUG_TEST);
-    gArgs.AddArg("-checklevel=<n>", strprintf("How thorough the block verification of -checkblocks is (0-4, default: %u)", DEFAULT_CHECKLEVEL), true, OptionsCategory::DEBUG_TEST);
+    gArgs.AddArg("-checklevel=<n>", strprintf("How thorough the block verification of -checkblocks is: "
+        "level 0 reads the blocks from disk, "
+        "level 1 verifies block validity, "
+        "level 2 verifies undo data, "
+        "level 3 checks disconnection of tip blocks, "
+        "and level 4 tries to reconnect the blocks, "
+        "each level includes the checks of the previous levels "
+        "(0-4, default: %u)", DEFAULT_CHECKLEVEL), true, OptionsCategory::DEBUG_TEST);
     gArgs.AddArg("-checkblockindex", strprintf("Do a full consistency check for mapBlockIndex, setBlockIndexCandidates, chainActive and mapBlocksUnlinked occasionally. (default: %u, regtest: %u)", defaultChainParams->DefaultConsistencyChecks(), regtestChainParams->DefaultConsistencyChecks()), true, OptionsCategory::DEBUG_TEST);
     gArgs.AddArg("-checkmempool=<n>", strprintf("Run checks every <n> transactions (default: %u, regtest: %u)", defaultChainParams->DefaultConsistencyChecks(), regtestChainParams->DefaultConsistencyChecks()), true, OptionsCategory::DEBUG_TEST);
     gArgs.AddArg("-checkpoints", strprintf("Disable expensive verification for known chain history (default: %u)", DEFAULT_CHECKPOINTS_ENABLED), true, OptionsCategory::DEBUG_TEST);