Improve documentation for useWatch exact

[KilianB]

Is your feature request related to a problem? Please describe.
Every few months I come back to the documentation of useWatch -> exact I need to go into the source code to figure out what this property is doing.

The description isn't self explanatory.

This prop will enable an exact match for input name subscriptions."

Describe the solution you'd like

I would like to have a more detailed description which explains what a non exact match includes and how the return value of such a subscription looks like.

Ref: https://github.com/orgs/react-hook-form/discussions/8447

and

https://github.com/react-hook-form/react-hook-form/blob/7e5af8d090fd882f4865ea9975b8f6a13bc596f1/src/logic/shouldSubscribeByName.ts#L14-L18