Merge #19005: doc: Add documentation for 'checklevel' argument in 'verifychain' RPC…

501e6ab4e7 doc: Add documentation for 'checklevel' argument in 'verifychain' RPC call (Calvin Kim)

Pull request description:

  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.

ACKs for top commit:
  jonatack:
    ACK 501e6ab4e7 `git diff 292ed3c 501e6ab` shows only change since last review is the verifychain RPCHelpMan edit; rebuild and retested manually anyway
  MarcoFalke:
    ACK 501e6ab4e7 🚝

Tree-SHA512: 09239f79c25b5c3022b8eb1f76198ba681305d7e8775038e46becffe5f6a14c572e0c5d06b0723fe9d4a015ec42c9f7ca7b80a2a93df0b1b66f5a84a80eeeeb1

src/validation.cpp

@@ -76,6 +76,14 @@ static constexpr std::chrono::hours DATABASE_WRITE_INTERVAL{1};
 static constexpr std::chrono::hours DATABASE_FLUSH_INTERVAL{24};
 /** Maximum age of our tip for us to be considered current for fee estimation */
 static constexpr std::chrono::hours MAX_FEE_ESTIMATION_TIP_AGE{3};
+const std::vector<std::string> CHECKLEVEL_DOC {
+    "level 0 reads the blocks from disk",
+    "level 1 verifies block validity",
+    "level 2 verifies undo data",
+    "level 3 checks disconnection of tip blocks",
+    "level 4 tries to reconnect the blocks",
+    "each level includes the checks of the previous levels",
+};
 
 bool CBlockIndexWorkComparator::operator()(const CBlockIndex *pa, const CBlockIndex *pb) const {
     // First sort by most total work, ...