Use allow(unused_imports) instead of cfg(doc) for imports used only for intra-doc links

Author: Joshua Nelson

library/std/src/sys_common/poison.rs

@@ -3,7 +3,7 @@ use crate::fmt;
 use crate::sync::atomic::{AtomicBool, Ordering};
 use crate::thread;
 
-#[cfg(doc)]
+#[allow(unused_imports)] // for intra-doc links
 use crate::sync::{Mutex, RwLock};
 
 pub struct Flag {