Merge bitcoin#29624: doc: update NeedsRedownload() and nStatus comment

af9f987 doc: update NeedsRedownload() comment (Sjors Provoost)

Pull request description:

  Noticed two outdated comments while reviewing bitcoin#29370.

  Since bitcoin#21009 we no longer roll back the chain, when a user updates a pre-segwit node to a modern node. In this unlikely scenario we tell the user to `-reindex`.

  This PR updates a comment in `PopulateAndValidateSnapshot` to reflect that change. Ditto for the description of `nStatus` in `chain.h`.

ACKs for top commit:
  maflcko:
    re-ACK af9f987
  fjahr:
    ACK af9f987

Tree-SHA512: d590f1cff6823297764c863753ed5478b8933d503c43933902d50b449dfd852a02aeb318c072ad25d02e4c2583d7026cd176a10b0584292d6bbe381a063f5c45

src/chain.h

@@ -178,7 +178,7 @@ class CBlockIndex
     //! Verification status of this block. See enum BlockStatus
     //!
     //! Note: this value is modified to show BLOCK_OPT_WITNESS during UTXO snapshot
-    //! load to avoid the block index being spuriously rewound.
+    //! load to avoid a spurious startup failure requiring -reindex.
     //! @sa NeedsRedownload
     //! @sa ActivateSnapshot
     uint32_t nStatus GUARDED_BY(::cs_main){0};

src/validation.cpp

@@ -6014,7 +6014,7 @@ util::Result<void> ChainstateManager::PopulateAndValidateSnapshot(
         index = snapshot_chainstate.m_chain[i];
 
         // Fake BLOCK_OPT_WITNESS so that Chainstate::NeedsRedownload()
-        // won't ask to rewind the entire assumed-valid chain on startup.
+        // won't ask for -reindex on startup.
         if (DeploymentActiveAt(*index, *this, Consensus::DEPLOYMENT_SEGWIT)) {
             index->nStatus |= BLOCK_OPT_WITNESS;
         }