Add references to isnull in notnull docs and vice versa

pandas/core/common.py

@@ -121,6 +121,10 @@ def isnull(obj):
     isnulled : array-like of bool or bool
         Array or bool indicating whether an object is null or if an array is
         given which of the element is null.
+
+    See also
+    --------
+    pandas.notnull: boolean inverse of pandas.isnull    
     """
     return _isnull(obj)
 
@@ -268,6 +272,10 @@ def notnull(obj):
     isnulled : array-like of bool or bool
         Array or bool indicating whether an object is *not* null or if an array
         is given which of the element is *not* null.
+
+    See also
+    --------
+    pandas.isnull : boolean inverse of pandas.notnull
     """
     res = isnull(obj)
     if np.isscalar(res):

pandas/core/generic.py

@@ -2545,12 +2545,20 @@ def interpolate(self, method='linear', axis=0, limit=None, inplace=False,
     def isnull(self):
         """
         Return a boolean same-sized object indicating if the values are null
+
+        See also
+        --------
+        notnull : boolean inverse of isnull
         """
         return isnull(self).__finalize__(self)
 
     def notnull(self):
         """Return a boolean same-sized object indicating if the values are
         not null
+
+        See also
+        --------
+        isnull : boolean inverse of notnull
         """
         return notnull(self).__finalize__(self)