DOC clarify inplace operation section in 1.5 whats_new
xref https://github.com/pandas-dev/pandas/issues/47381

Coming from scikit-learn tests failing on scikit-learn with the pandas development version, I found the whats_new entry not very helpful at all. This does a few things:
- use an example with different dtypes so that the behaviour is as advertised for old behaviour (i.e. `ser` is not updated in place). This was not the case before, since when dtype matches, `ser` is updated in place so the code snippet was not showing the right behaviour.
- make it clearer that there is no behaviour change in pandas 1.5

Some of the wording may be not completely accurate, as I don't have a very good grasp of the pandas internals, feel free to suggest improvements!

I have tried to make the diff smaller as advised > If you want to spend the time, I think it'd help read this section is the example had some more "interesting" data. 

I did this in my last commit, I also showed `df.iloc[:, 0]` which I think helps in understanding what is happening (whether the column keeps its dtype or not). Thanks @lesteve 