DOC: update the pandas.DataFrame.isna and pandas.Series.isna docstring (#20138)

Author: Donk23

pandas/core/generic.py

@@ -5585,13 +5585,63 @@ def asof(self, where, subset=None):
     # Action Methods
 
     _shared_docs['isna'] = """
+        Detect missing values.
+
         Return a boolean same-sized object indicating if the values are NA.
+        NA values, such as None or :attr:`numpy.NaN`, gets mapped to True
+        values.
+        Everything else gets mapped to False values. Characters such as empty
+        strings `''` or :attr:`numpy.inf` are not considered NA values
+        (unless you set ``pandas.options.mode.use_inf_as_na = True``).
+
+        Returns
+        -------
+        %(klass)s
+            Mask of bool values for each element in %(klass)s that
+            indicates whether an element is not an NA value.
 
         See Also
         --------
-        %(klass)s.notna : boolean inverse of isna
         %(klass)s.isnull : alias of isna
+        %(klass)s.notna : boolean inverse of isna
+        %(klass)s.dropna : omit axes labels with missing values
         isna : top-level isna
+
+        Examples
+        --------
+        Show which entries in a DataFrame are NA.
+
+        >>> df = pd.DataFrame({'age': [5, 6, np.NaN],
+        ...                    'born': [pd.NaT, pd.Timestamp('1939-05-27'),
+        ...                             pd.Timestamp('1940-04-25')],
+        ...                    'name': ['Alfred', 'Batman', ''],
+        ...                    'toy': [None, 'Batmobile', 'Joker']})
+        >>> df
+           age       born    name        toy
+        0  5.0        NaT  Alfred       None
+        1  6.0 1939-05-27  Batman  Batmobile
+        2  NaN 1940-04-25              Joker
+
+        >>> df.isna()
+             age   born   name    toy
+        0  False   True  False   True
+        1  False  False  False  False
+        2   True  False  False  False
+
+        Show which entries in a Series are NA.
+
+        >>> ser = pd.Series([5, 6, np.NaN])
+        >>> ser
+        0    5.0
+        1    6.0
+        2    NaN
+        dtype: float64
+
+        >>> ser.isna()
+        0    False
+        1    False
+        2     True
+        dtype: bool
         """
 
     @Appender(_shared_docs['isna'] % _shared_doc_kwargs)