selinux: add the desc of the module

Author: sylvestre

src/uucore/src/lib/features/selinux.rs

@@ -3,6 +3,8 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
+//! Set of functions to manage SELinux security contexts
+
 use std::error::Error;
 use std::path::Path;