docs: Correct `useFormStatus` to be a client component (#50991)
### What?
The `useFormStatus` needs to be used in a client component. That
component should be used within a `form` for the `pending` property to
reflect the form status.
### Why?
The docs are currently not accurate.
### Notes
I could also update the name of the file to not be `form.js`, which
implies this is the entire form instead of just the submit button being
used as a component within the form.
