Add reasoning in whatsnew

Author: Aloqeely

doc/source/whatsnew/v3.0.0.rst

@@ -28,6 +28,8 @@ New Deprecation Policy
 ^^^^^^^^^^^^^^^^^^^^^^
 pandas 3.0.0 introduces a new 3-stage deprecation policy: using ``DeprecationWarning`` initially, then switching to ``FutureWarning`` for broader visibility in the last minor version before the next major release, and then removal of the deprecated functionality in the major release.
 
+This was done to give downstream packages more time to adjust to pandas deprecations, which should reduce the amount of warnings that a user gets from code that isn't theirs.
+
 .. _whatsnew_300.enhancements.other:
 
 Other enhancements