doc: add note about confusing HaveTxsDownloaded name

Author: jamesob

src/chain.h

@@ -276,6 +276,12 @@ class CBlockIndex
      *
      * Does not imply the transactions are consensus-valid (ConnectTip might fail)
      * Does not imply the transactions are still stored on disk. (IsBlockPruned might return true)
+     *
+     * Note that this will be true for the snapshot base block, if one is loaded (and
+     * all subsequent assumed-valid blocks) since its nChainTx value will have been set
+     * manually based on the related AssumeutxoData entry.
+     *
+     * TODO: potentially change the name of this based on the fact above.
      */
     bool HaveTxsDownloaded() const { return nChainTx != 0; }