Improve documentation of isEqual prop on Field

### Are you submitting a **bug report** or a **feature request**?

Could be either, I guess - improve docs (feature) because there is missing info (bug).

### What is the current behavior?

The doc for FieldProps doesn't explain the use and purpose for `isEqual`.

### What is the expected behavior?

The doc should describe how `isEqual` is used in the Field component and explain the intended purpose of the prop. See [FieldProps - parse](https://final-form.org/docs/react-final-form/types/FieldProps#parse) for an example of what I mean.

### Website Link

[FieldProps - isEqual](https://final-form.org/docs/react-final-form/types/FieldProps#isequal)

### What's your environment?

N/A? Current website as of 2020-08-28 viewed on Chrome.

I've looked through the code some and done a few searches and still haven't figured out for sure how it works - otherwise I'd just submit a PR for the change. It doesn't seem to be used for calculating form updates, though, which is what I was hoping for when I found it.