docs: clarify data type in Forms and Mutations section (#54630)
### What?
Clarify the "Forms and Mutations" docs.
### Why?
This may be opinionated, but I believe this section of the docs starts out assuming that the user knows how the `<form>` HTML works. Coming from the client side React world, most developers are used to using state in forms. Clarifying that this data is accessible via the standard web API [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData/FormData) is essential for both transitioning developers and new developers.
