ENH: pandas-dev#14591 - tests and documentation

Author: nchmura4

pandas/core/generic.py

@@ -4003,6 +4003,10 @@ def asfreq(self, freq, method=None, how=None, normalize=False, fill_value=None):
             For PeriodIndex only, see PeriodIndex.asfreq
         normalize : bool, default False
             Whether to reset output index to midnight
+        fill_value: scalar, optional
+            value to use for missing values, applied during upsampling
+
+            .. version added:: 0.20.0
 
         Returns
         -------