Documentation: kunit: Cleanup run_wrapper, fix x-ref

The "Run Tests on qemu" section of run_wrapper.rst had a few issues left
over from the last big documentation refactor[1]:
- It referenced a non_uml.rst page, which was integrated into the other
  pages (including run_wrapper.rst).
- It skimmed over the use of --arch= and --cross_compile= in favour of
  using a custom --qemu_config. Since most users will want to use the
  former, let's give examples.

Remove the reference to the non-existant page, and add a couple of
examples to encourage the use of --arch= and --cross_compile=.

With this change, there should be no more broken references in the KUnit
documentation (i.e., the one mentioned in [2] is gone).

[1]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=46201d47


[2]: https://lore.kernel.org/linux-doc/cover.1656234456.git.mchehab@kernel.org/

Signed-off-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>