add section about string dtype

Author: jorisvandenbossche

doc/source/whatsnew/v3.0.0.rst

@@ -14,10 +14,57 @@ including other versions of pandas.
 Enhancements
 ~~~~~~~~~~~~
 
-.. _whatsnew_300.enhancements.enhancement1:
+.. _whatsnew_300.enhancements.string_dtype:
+
+Dedicated string data type by default
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Historically, pandas represented string columns with NumPy ``object`` data type.
+This representation has numerous problems: it is not specific to strings (any
+Python object can be stored in an ``object``-dtype array, not just strings) and
+it is often not very efficient (both performance wise and for memory usage).
+
+Starting with pandas 3.0, a dedicated string data type is enabled by default
+(backed by PyArrow under the hood, if installed, otherwise falling back to
+NumPy). This means that pandas will start inferring columns containing string
+data as the new ``str`` data type when creating pandas objects, such as in
+constructors or IO functions.
+
+Old behavior:
+
+.. code-block:: python
+
+    >>> ser = pd.Series(["a", "b"])
+    0    a
+    1    b
+    dtype: object
+
+New behavior:
+
+.. code-block:: python
+
+    >>> ser = pd.Series(["a", "b"])
+    0    a
+    1    b
+    dtype: str
+
+The string data type that is used in these scenarios will mostly behave as NumPy
+object would, including missing value semantics and general operations on these
+columns.
+
+The main characteristic of the new string data type:
+
+- Inferred by default for string data (instead of object dtype)
+- The ``str`` dtype can only hold strings (or missing values), in contrast to
+  ``object`` dtype. (setitem with non string fails)
+- The missing value sentinel is always ``NaN`` (``np.nan``) and follows the same
+  missing value semantics as the other default dtypes.
+
+Those intentional changes can have breaking consequences, for example when checking
+for the ``.dtype`` being object dtype or checking the exact missing value sentinel.
+
+TODO add link to migration guide for more details
 
-Enhancement1
-^^^^^^^^^^^^
 
 .. _whatsnew_300.enhancements.enhancement2: