whatsnew doc about changed integer conversion

Author: jbrockmendel

doc/source/whatsnew/v3.0.0.rst

@@ -374,6 +374,8 @@ In cases with mixed-resolution inputs, the highest resolution is used:
     In [2]: pd.to_datetime([pd.Timestamp("2024-03-22 11:43:01"), "2024-03-22 11:43:01.002"]).dtype
     Out[2]: dtype('<M8[ns]')
 
+Many users will now get "M8[us]" dtype  data in cases when they used to get "M8[ns]". For most use cases they should not notice a difference. One big exception is converting to integers, which will give integers 1000x smaller.
+
 .. _whatsnew_300.api_breaking.value_counts_sorting:
 
 Changed behavior in :meth:`DataFrame.value_counts` and :meth:`DataFrameGroupBy.value_counts` when ``sort=False``