doc: Add and edit some comments around assumeutxo

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

src/node/blockstorage.cpp

@@ -761,9 +761,10 @@ bool BlockManager::FlushChainstateBlockFile(int tip_height)
 {
     LOCK(cs_LastBlockFile);
     auto& cursor = m_blockfile_cursors[BlockfileTypeForHeight(tip_height)];
+    // If the cursor does not exist, it means an assumeutxo snapshot is loaded,
+    // but no blocks past the snapshot height have been written yet, so there
+    // is no data associated with the chainstate, and it is safe not to flush.
     if (cursor) {
-        // The cursor may not exist after a snapshot has been loaded but before any
-        // blocks have been downloaded.
         return FlushBlockFile(cursor->file_num, /*fFinalize=*/false, /*finalize_undo=*/false);
     }
     return false;