rust: kernel: add docs for macro stub

So that we can enable `-Wmissing_docs` globally.

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

Author: ojeda

rust/kernel/print.rs

@@ -190,7 +190,7 @@ macro_rules! print_macro (
     );
 );
 
-// Stub for doctests
+/// Stub for doctests
 #[cfg(testlib)]
 #[macro_export]
 macro_rules! print_macro (