rust: kernel: start using doc_cfg and doc

As an example, use `doc(cfg(...))` and `cfg(doc)` to gate `sysctl`.

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

rust/kernel/lib.rs

@@ -64,7 +64,8 @@ pub mod random;
 mod static_assert;
 pub mod sync;
 
-#[cfg(CONFIG_SYSCTL)]
+#[cfg(any(CONFIG_SYSCTL, doc))]
+#[doc(cfg(CONFIG_SYSCTL))]
 pub mod sysctl;
 
 pub mod io_buffer;