comment out not-ready section title

Author: jorisvandenbossche

doc/source/user_guide/migration-3-strings.rst

@@ -96,7 +96,8 @@ non-string value in it (see below for more details).
 Missing values with the new string dtype are always represented as ``NaN`` (``np.nan``),
 and the missing value behavior is similar to other default dtypes.
 
-This new string dtype should otherwise behave the same as the existing ``object`` dtype users are used to.  For example, all string-specific methods
+This new string dtype should otherwise behave the same as the existing
+``object`` dtype users are used to. For example, all string-specific methods
 through the ``str`` accessor will work the same:
 
 .. code-block:: python
@@ -379,7 +380,7 @@ raise an error regardless of the number of strings:
    ...
    TypeError: Cannot perform reduction 'prod' with string dtype
 
-For existing users of the nullable ``StringDtype``
---------------------------------------------------
+.. For existing users of the nullable ``StringDtype``
+.. --------------------------------------------------
 
-TODO
+.. TODO