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.