address concerns - update description

Signed-off-by: Kareem Farid <[email protected]>

openlane/steps/openroad.py

@@ -1351,7 +1351,7 @@ class _GlobalPlacement(OpenROADStep):
             Variable(
                 "PL_KEEP_RESIZE_BELOW_OVERFLOW",
                 Optional[Decimal],
-                "Only applicable when PL_TIME_DRIVEN is enabled. When the overflow is below the set value, timing-driven iterations will retain the resizer changes instead of reverting them. Allowed values are 0 to 1.",
+                "Only applicable when PL_TIME_DRIVEN is enabled. When the overflow is below the set value, timing-driven iterations will retain the resizer changes instead of reverting them. Allowed values are 0 to 1. If not set, a nonzero default value from OpenROAD will be used",
             ),
         ]
     )