validation: Mark g_chainman DEPRECATED

src/validation.h

@@ -873,6 +873,7 @@ public:
     void Reset();
 };
 
+/** DEPRECATED! Please use node.chainman instead. May only be used in validation.cpp internally */
 extern ChainstateManager g_chainman GUARDED_BY(::cs_main);
 
 /** @returns the most-work valid chainstate. */