DOC: Add note for copy keywords for Copy-on-Write

pandas/core/generic.py

@@ -6428,6 +6428,15 @@ def astype(
             Return a copy when ``copy=True`` (be very careful setting
             ``copy=False`` as changes to values then may propagate to other
             pandas objects).
+
+            .. note::
+                The copy-keyword will change behavior in pandas 3.0.
+                `Copy-on-Write
+                <https://pandas.pydata.org/docs/dev/user_guide/copy_on_write.html>`__
+                will be enabled by default, which means that all methods with a
+                copy-keyword will use a lazy copy mechanism and ignore the copy
+                keyword. The copy-keyword will be removed in a future version of
+                pandas.
         errors : {'raise', 'ignore'}, default 'raise'
             Control raising of exceptions on invalid data for provided dtype.