message: doc: nChainTx needs to become a 64-bit earlier due to SegWit
diff: @@ -181,7 +181,7 @@ public:
  //! (memory only) Number of transactions in the chain up to and including this block.
  //! This value will be non-zero only if and only if transactions for this block and all its parents are available.
  - //! Change to 64-bit type when necessary; won't happen before 2030
  + //! Change to 64-bit type before 2024 (assuming worst case of 60 byte transactions).
  //!
  //! Note: this value is faked during use of a UTXO snapshot because we don't
  //! have the underlying block data available during snapshot load.
  