Add whatsnew entry for mean

Author: kinshukdua

doc/source/whatsnew/v1.4.0.rst

@@ -240,10 +240,38 @@ Now the float-dtype is respected. Since the common dtype for these DataFrames is
 
     res
 
-.. _whatsnew_140.notable_bug_fixes.notable_bug_fix3:
+.. _whatsnew_140.notable_bug_fixes.string_to_numeric_in_mean:
 
-notable_bug_fix3
-^^^^^^^^^^^^^^^^
+Implicit conversion of string to numeric type in mean
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+When computing the ``mean`` of a :class:`Series` or :class:`DataFrame` with a string-type value, the elements are concatenated
+to a single string then coerced to a numeric type implicitly before computing the mean. This can lead to unexpected results:
+
+.. code-block:: ipython
+
+    In [5]: df = DataFrame({
+                    "A": ["1", "2", "3"],
+                    "B": ["0", "1", "J"],
+                })
+    In [6]: df.mean(numeric_only=False)
+    Out[6]:
+    A     41.00000+0.00000j
+    C    0.000000+0.333333j
+    dtype: complex128
+
+Now, an exception will be raised whenever ``mean`` is called on string-type columns or :class:`Series`.
+
+.. code-block:: ipython
+
+    In [7]: df = DataFrame({
+                    "A": ["1", "2", "3"],
+                    "B": ["0", "1", "J"],
+                })
+    In [8]: df.mean(numeric_only=False)
+    Out[8]:
+    ...
+    TypeError: cannot find the mean of type 'str'
 
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