ENH: add inplace-kwarg to update
Currently, `df.update` follows the convention of `dict.update` to return None and update inplace. This is against the prevailing trend (and philosopy?) of pandas to move away from `inplace`. See for example (one among many...) @TomAugspurger's response in #21841:
> Generally, we're moving away from inplace operations. It's confusing whether inplace means no copy or not. Reindex, by definition, can't be inplace unless the index is the same.
We recommend chaining your method calls, and hope to provide better memory control in the future.

The `update`-method of perfoms an important function regardless of whether it returns the object or None, and so should IMO be enabled to work in chained operations as well.

First step there would be adding an `inplace`-argument with default `True`, and then -- *potentially* -- transitioning with a longish deprecation cycle towards `inplace=False`.

Relevant xrefs: #21855 #21859