Documentation: rust: testing: add Kconfig guidance

Documentation: rust: testing: add Kconfig guidance

Now that the Rust KUnit tests are protected with Kconfig, update the
documentation to mention it.

Signed-off-by: Yury Norov <ynorov@nvidia.com>
Reviewed-by: David Gow <david@davidgow.net>
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260417031531.315281-4-ynorov@nvidia.com
[ Fixed the paragraph by moving the new sentence above. Added gate
  in the other example as well. Applied proper formatting. Reworded
  slightly. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>