[Emam2006]

.item() on a lazy (dask-backed) DataArray/Variable currently raises a
generic NotImplementedError with no hint about how to proceed (#7916).

Per the discussion in #7916, the agreed resolution is not to implicitly
compute, but to document the limitation and make the error actionable. This PR:

• improves the NotImplementedError raised for methods that don't work on dask
  arrays (fail_on_dask_array_input) to suggest loading the data into memory
  first, e.g. with .compute() or .load();
• adds a note to the DataArray.item / Variable.item docstring pointing to
  .compute().item() / .as_numpy().item() (as suggested by @dcherian and
  @slevang in the thread);
• adds a whats-new.rst entry.

No behaviour change — eager .item() works exactly as before; only the error
message text and docstring change.

Checklist

• Closes .item() breaks on dataarray if dask arrays are (implicitely) used #7916
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.
    Tools: Claude Code (Claude Opus 4.8). Prompt: implement the maintainer-agreed
    resolution of .item() breaks on dataarray if dask arrays are (implicitely) used #7916 — document that .item() is not implemented for lazy
    (dask) arrays and make the raised error message suggest loading into memory
    first; keep it docs/message-only with no behaviour change.

[headtr1ck]

See https://xray--11659.org.readthedocs.build/en/11659/generated/xarray.DataArray.item.html#xarray.DataArray.item

There is already a "Notes" section. I guess we should move the extra note into there?