Explain the _oibit_remover field

src/libcore/fmt/mod.rs

@@ -261,6 +261,13 @@ pub struct Formatter<'a> {
 
 struct Void {
  _priv: (),
+ /// Erases all oibits, because `Void` erases the type of the object that
+ /// will be used to produce formatted output. Since we do not know what
+ /// oibits the real types have (and they can have any or none), we need to
+ /// take the most conservative approach and forbid all oibits.
+ ///
+ /// It was added after #45197 showed that one could share a `!Sync`
+ /// object across threads by passing it into `format_args!`.
  _oibit_remover: PhantomData<*mut Fn()>,
 }