Unclear docs on form.watch callback variant

[bryanhaney-shipengine]

It took some head scratching and source diving to figure out why the callback variant of form.watch(() => { ... }) was getting called increasingly more times on every field change. The docs are not clear that the callback variant sets up a new subscription per call, and thus it belongs in a useEffect.

[bluebill1049]

Did you miss the example code below?

https://react-hook-form.com/api/useform/watch

send us a PR if you have better suggestion to improve on it.