DOC: Fix groupby nth (#13810)

Author: sinhrks

pandas/core/groupby.py

@@ -1205,32 +1205,55 @@ def nth(self, n, dropna=None):
 
         Examples
         --------
-        >>> df = DataFrame([[1, np.nan], [1, 4], [5, 6]], columns=['A', 'B'])
+
+        >>> df = pd.DataFrame({'A': [1, 1, 2, 1, 2],
+        ...                    'B': [np.nan, 2, 3, 4, 5]}, columns=['A', 'B'])
         >>> g = df.groupby('A')
         >>> g.nth(0)
-           A   B
-        0  1 NaN
-        2  5   6
+             B
+        A
+        1  NaN
+        2  3.0
         >>> g.nth(1)
-           A  B
-        1  1  4
+             B
+        A
+        1  2.0
+        2  5.0
         >>> g.nth(-1)
-           A  B
-        1  1  4
-        2  5  6
+             B
+        A
+        1  4.0
+        2  5.0
+        >>> g.nth([0, 1])
+             B
+        A
+        1  NaN
+        1  2.0
+        2  3.0
+        2  5.0
+
+        Specifying ``dropna`` allows count ignoring NaN
+
         >>> g.nth(0, dropna='any')
-           B
-           A
-        1  4
-        5  6
+             B
+        A
+        1  2.0
+        2  3.0
 
         NaNs denote group exhausted when using dropna
 
-        >>> g.nth(1, dropna='any')
+        >>> g.nth(3, dropna='any')
             B
-            A
+        A
         1 NaN
-        5 NaN
+        2 NaN
+
+        Specifying ``as_index=False`` in ``groupby`` keeps the original index.
+
+        >>> df.groupby('A', as_index=False).nth(1)
+           A    B
+        1  1  2.0
+        4  2  5.0
         """
 
         if isinstance(n, int):