PyNIO deprecation warning in docs

[lsterzinger]

What is your issue?

Noticed this incomplete warning about PyNIO deprecation

xarray/doc/user-guide/io.rst

Line 1280 in 5043223

The PyNIO backend is deprecated_. PyNIO is no longer maintained_. See

https://docs.xarray.dev/en/stable/user-guide/io.html?highlight=pynio#formats-supported-by-pynio

Was there a resource that should be mentioned here?