add clarification in comment

Author: marcofleon

src/net_processing.h

@@ -74,7 +74,8 @@ class PeerManager : public CValidationInterface, public NetEventsInterface
         //! Whether or not the internal RNG behaves deterministically (this is
         //! a test-only option).
         bool deterministic_rng{false};
-        //! Number of headers sent in one getheaders message result.
+        //! Number of headers sent in one getheaders message result (this is
+        //! a test-only option).
         uint32_t max_headers_result{MAX_HEADERS_RESULTS};
     };