DOC: Clarify difference between StringDtype(pyarrow) and ArrowDtype(string)
Spawned from a discussion in #52156

Is there a plan to deprecate the older one and if so should that be mentioned here? thx @mroeschke 

There is a discussion about deprecating this somewhere but no consensus yet. Yeah the discussion started at https://github.com/pandas-dev/pandas/issues/48469