[doxygen] Actually display comment

src/main.h

@@ -149,7 +149,7 @@ extern CFeeRate minRelayTxFee;
 /** Absolute maximum transaction fee (in satoshis) used by wallet and mempool (rejects high fee in sendrawtransaction) */
 extern CAmount maxTxFee;
 extern bool fAlerts;
-/* If the tip is older than this (in seconds), the node is considered to be in initial block download. */
+/** If the tip is older than this (in seconds), the node is considered to be in initial block download. */
 extern int64_t nMaxTipAge;
 extern bool fPermitReplacement;