validateSync middleware cycle

Is this intended or a bug?
```js
schema.pre('validate', () => ++pre);
schema.post('validate', () => ++post);

doc.validateSync(); // pre/post stay 0
await doc.validate(); // pre/post run
```

`doc.validateSync()` has a `middleware: false` option added in #15883 , but there's not validateSync middleware cycle at all right now.
I should either remove the option from the docs/TS, or add a validateSync synchronous middleware akin to `init`.

Thoughts? @vkarpov15 @hasezoey

Start with the doc.validateSync() entry point and its middleware: false option, then compare the existing async validate() path with the synchronous init middleware mentioned in the issue. Check the documentation and TypeScript definitions referenced in the issue; done means the chosen validateSync middleware behavior is implemented consistently or the option is removed from both places.

Written by the indexing model from the issue text.