doc: Add doxygen comment to IsRBFOptIn

src/policy/rbf.h

@@ -7,16 +7,27 @@
 
 #include <txmempool.h>
 
+/** The rbf state of unconfirmed transactions */
 enum class RBFTransactionState {
+ /** Unconfirmed tx that does not signal rbf and is not in the mempool */
  UNKNOWN,
+ /** Either this tx or a mempool ancestor signals rbf */
  REPLACEABLE_BIP125,
- FINAL
+ /** Neither this tx nor a mempool ancestor signals rbf */
+ FINAL,
 };
 
-// Determine whether an in-mempool transaction is signaling opt-in to RBF
-// according to BIP 125
-// This involves checking sequence numbers of the transaction, as well
-// as the sequence numbers of all in-mempool ancestors.
+/**
+ * Determine whether an unconfirmed transaction is signaling opt-in to RBF
+ * according to BIP 125
+ * This involves checking sequence numbers of the transaction, as well
+ * as the sequence numbers of all in-mempool ancestors.
+ *
+ * @param tx The unconfirmed transaction
+ * @param pool The mempool, which may contain the tx
+ *
+ * @return The rbf state
+ */
 RBFTransactionState IsRBFOptIn(const CTransaction& tx, const CTxMemPool& pool) EXCLUSIVE_LOCKS_REQUIRED(pool.cs);
 RBFTransactionState IsRBFOptInEmptyMempool(const CTransaction& tx);