DOC: update docstring for series.nunique (#25116)

Author: cecilialiao

pandas/core/base.py

@@ -1323,12 +1323,31 @@ def nunique(self, dropna=True):
 
         Parameters
         ----------
-        dropna : boolean, default True
+        dropna : bool, default True
             Don't include NaN in the count.
 
         Returns
         -------
-        nunique : int
+        int
+
+        See Also
+        --------
+        DataFrame.nunique: Method nunique for DataFrame.
+        Series.count: Count non-NA/null observations in the Series.
+
+        Examples
+        --------
+        >>> s = pd.Series([1, 3, 5, 7, 7])
+        >>> s
+        0    1
+        1    3
+        2    5
+        3    7
+        4    7
+        dtype: int64
+
+        >>> s.nunique()
+        4
         """
         uniqs = self.unique()
         n = len(uniqs)