message: doc: add comment for g_best_block
diff: @@ -101,6 +101,7 @@ extern RecursiveMutex cs_main;
  typedef std::unordered_map<uint256, CBlockIndex*, BlockHasher> BlockMap;
  extern Mutex g_best_block_mutex;
  extern std::condition_variable g_best_block_cv;
  +/** Used to notify getblocktemplate RPC of new tips. */
  extern uint256 g_best_block;
  /** Whether there are dedicated script-checking threads running.
  * False indicates all script checking is done on the main threadMessageHandler thread.
  