doc: nChainTx needs to become a 64-bit earlier due to SegWit

Author: Sjors

src/chain.h

@@ -170,7 +170,7 @@ class CBlockIndex
 
     //! (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.