doc: add comment about CCoinsViewDBCursor constructor

src/txdb.cpp

@@ -172,6 +172,8 @@ bool CBlockTreeDB::ReadLastBlockFile(int &nFile) {
 class CCoinsViewDBCursor: public CCoinsViewCursor
 {
 public:
+    // Prefer using CCoinsViewDB::Cursor() since we want to perform some
+    // cache warmup on instantiation.
     CCoinsViewDBCursor(CDBIterator* pcursorIn, const uint256&hashBlockIn):
         CCoinsViewCursor(hashBlockIn), pcursor(pcursorIn) {}
     ~CCoinsViewDBCursor() {}