DOC: add deprecation of chained assignment to 2.2 whatsnew
Part of the warnings we have been adding in https://github.com/pandas-dev/pandas/issues/56019, specifically the ones for chained assignment / chained inplace methods, are already being raised in the default mode, because this is a part of the changes with CoW we can clearly deprecate in advance. 
Therefore adding a whatsnew section about this.

thx @jorisvandenbossche  