Documentation: rust: rename `#[test]`s to "`rusttest` host tests"

Now that `rusttest`s are not really used much, clarify the section of
the documentation that describes them.

In addition, free the section name for the KUnit-based `#[test]`s that
will be added afterwards. To do so, rename the section into `rusttest`
host tests.

Reviewed-by: David Gow <davidgow@google.com>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/r/20250502215133.1923676-7-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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