docs: rust: include other expressions in conditional compilation section

Expand the conditional compilation section to explain how to support
other expressions, such as testing whether `RUSTC_VERSION` is at least
a given version, which requires a numerical comparison that Rust's `cfg`
predicates do not support (yet?).

Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
Tested-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20240902165535.1101978-7-ojeda@kernel.org


Signed-off-by: Miguel Ojeda <ojeda@kernel.org>