Clarifying `from_err_ptr` may return `Ok(NULL)`

[ojeda]

Please read https://lore.kernel.org/rust-for-linux/20260322193830.89324-1-ojeda@kernel.org/ for context -- this issue is about this part:

  - On the Rust general infrastructure side, we should document more
    prominently that `from_err_ptr` may return `Ok(NULL)` just fine,
    since it calls `IS_ERR`. It may be obvious, since `NULL` is not an
    error value, but still, it could have perhaps prevented this issue.

    We should also include an example to the doctest showing and testing
    that particular case to drive the point home.

Regarding the doctest part: it should be a new one, or the existing one should be made non-ignore.

In addition, the example should assert not just for Ok(NULL), but also other cases.

---

This requires submitting a proper patch to the LKML and the Rust for Linux mailing list. Please recall to test your changes (including generating the documentation if changed, running the Rust doctests if changed, etc.), to use a proper title for the commit, to sign your commit under the Developer's Certificate of Origin and to add a Suggested-by: tag, and a Link: tag to this issue. Please see https://docs.kernel.org/process/submitting-patches.html and https://rust-for-linux.com/contributing for details.