DOC: update copy keyword explanation in astype docstring (#63274)

Author: jorisvandenbossche

pandas/core/generic.py

@@ -6354,23 +6354,18 @@ def astype(
             a numpy.dtype or Python type to cast one or more of the DataFrame's
             columns to column-specific types.
         copy : bool, default False
-            Return a copy when ``copy=True`` (be very careful setting
-            ``copy=False`` as changes to values then may propagate to other
-            pandas objects).
+            This keyword is now ignored; changing its value will have no
+            impact on the method.
 
-            .. 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.
+            .. deprecated:: 3.0.0
 
-                You can already get the future behavior and improvements through
-                enabling copy on write ``pd.options.mode.copy_on_write = True``
+                This keyword is ignored and will be removed in pandas 4.0. Since
+                pandas 3.0, this method always returns a new object using a lazy
+                copy mechanism that defers copies until necessary
+                (Copy-on-Write). See the `user guide on Copy-on-Write
+                <https://pandas.pydata.org/docs/dev/user_guide/copy_on_write.html>`__
+                for more details.
 
-            .. deprecated:: 3.0.0
         errors : {'raise', 'ignore'}, default 'raise'
             Control raising of exceptions on invalid data for provided dtype.