Workaround for rustdoc bug in new beta

Filed rust-lang#122758 to track a proper fix, but this seems to solve the
problem in the meantime and is probably OK in terms of impact on
(internal) doc quality.

compiler/rustc_data_structures/src/sync/lock.rs

@@ -189,6 +189,7 @@ mod no_sync {
     use super::Mode;
     use std::cell::RefCell;
 
+    #[doc(no_inline)]
     pub use std::cell::RefMut as LockGuard;
 
     pub struct Lock<T>(RefCell<T>);