MINOR: mux-quic: correct qcc_bufwnd_full() documentation

Fix returned value domment of qcc_bufwnd_full() which was incorrect.

src/mux_quic.c

@@ -524,7 +524,7 @@ void qcs_notify_send(struct qcs *qcs)
 	}
 }
 
-/* Returns true if a Tx stream buffer can be allocated. */
+/* Returns true if <qcc> buffer window does not have room for a new buffer. */
 static inline int qcc_bufwnd_full(const struct qcc *qcc)
 {
 	const struct quic_conn *qc = qcc->conn->handle.qc;