more details about -gwnum option

INSTALL-gwnum

@@ -152,7 +152,8 @@ Step 2 took 525ms
    particular test:
    
      "-gwnum" will override the thresholds in ecm-impl.h and force gwnum
-     routines to be used as long as the input number is greater than 2^350.
+     routines to be always used (for input numbers greater than 2^350,
+     which is the gwnum own threshold).
      
      "-no-gwnum" requires no explanation.