docs: rust: Add description of Rust documentation test as KUnit ones

Rust documentation tests are automatically converted into KUnit
tests. The commit adding this feature

commit a66d733d

 ("rust: support running Rust documentation tests as KUnit ones")

from Miguel has a very nice commit message with a lot details
for this. To not 'hide' that just in a commit message, pick the main
parts of it and add it to the documentation. And add a short info
how to enable this. While adding this, improve the structure of
the sections.

Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Co-developed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Reviewed-by: Trevor Gross <tmgross@umich.edu>
Link: https://lore.kernel.org/r/20240130075117.4137360-2-dirk.behme@de.bosch.com
[ Fixed unordered list rendering, rewrapped text and made headers
  consistent with the other documents in `rust/`. ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>