doc: Add cs_main lock annotations for mapBlockIndex

src/validation.cpp

@@ -152,7 +152,7 @@ private:
 
 public:
     CChain chainActive;
-    BlockMap mapBlockIndex;
+    BlockMap mapBlockIndex GUARDED_BY(cs_main);
     std::multimap<CBlockIndex*, CBlockIndex*> mapBlocksUnlinked;
     CBlockIndex *pindexBestInvalid = nullptr;