DOC: ddof in cov when nans present (#52137)

Author: ma-ma-hima

pandas/core/frame.py

@@ -10557,6 +10557,7 @@ def cov(
         ddof : int, default 1
             Delta degrees of freedom.  The divisor used in calculations
             is ``N - ddof``, where ``N`` represents the number of elements.
+            This argument is applicable only when no ``nan`` is in the dataframe.
 
             .. versionadded:: 1.1.0