rust: rework `build_assert!` documentation

rust: rework `build_assert!` documentation

Add a detailed comparison and recommendation of the three types of
build-time assertion macro as module documentation (and un-hide the module
to render them).

The documentation on the macro themselves are simplified to only cover the
scenarios where they should be used; links to the module documentation is
added instead.

Reviewed-by: Yury Norov <ynorov@nvidia.com>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Alexandre Courbot <acourbot@nvidia.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Danilo Krummrich <dakr@kernel.org>
Link: https://patch.msgid.link/20260319121653.2975748-4-gary@kernel.org
[ Added periods on comments. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>