DOC: Add note for copy keywords for Copy-on-Write (#56033)

Author: phofl

pandas/core/generic.py

@@ -6429,6 +6429,18 @@ 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 to defer the copy and
+                ignore the `copy` keyword. The `copy` keyword will be removed in a
+                future version of pandas.
+
+                You can already get the future behavior and improvements through
+                enabling copy on write ``pd.options.mode.copy_on_write = True``
         errors : {'raise', 'ignore'}, default 'raise'
             Control raising of exceptions on invalid data for provided dtype.