Improve thread safety docs/bounds for PrimitiveMut

PiperOrigin-RevId: 591047379

rust/primitive.rs

@@ -37,6 +37,8 @@ impl<'msg, T> PrimitiveMut<'msg, T> {
     }
 }
 
+// SAFETY: all `T` that can perform mutations don't mutate through a shared
+// reference.
 unsafe impl<'msg, T> Sync for PrimitiveMut<'msg, T> {}
 
 impl<'msg, T> PrimitiveMut<'msg, T>

rust/vtable.rs

@@ -432,6 +432,8 @@ pub trait PrimitiveWithRawVTable:
     + Debug
     + 'static
     + ProxiedWithPresence
+    + Sync
+    + Send
     + for<'msg> Proxied<View<'msg> = Self, Mut<'msg> = PrimitiveMut<'msg, Self>>
 {
 }