DOC: [ArrowStringArray] release note and other documentation
extremely draft status. placeholder for discussion.

we may want to combine this with #39908, and merge #40708 before #39908

#39908 makes changes to the user facing api that affects the existing StringDtype so that may need an separate section in the release notes.

other sections of the documentation may also need updating. e.g. doc/source/user_guide/text.rst, doc/source/development/roadmap.rst and doc/source/user_guide/basics.rst
not sure if we want to do anything with `df.convert_dtypes()`, maybe `convert_string` parameter could take an additional type to allow conversion to pyarrow backed nullable string array  will combine this with #39908 as that PR makes the dtype public