doc: add comment for g_best_block

Author: jamesob

src/validation.h

@@ -109,6 +109,7 @@ 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.