Doc tweaks: Arrange FormState's properties in cagtegories

[honey32]

Hello.
I really appreciate this lib's reasonable and simple design.
Let me contribute on improving docs for our better understanding and wider spreading.

Background

I have difficulty on understanding the outline of the FormState type document.
Especially, it is very hard to visualize in mind the relation between each properties

for example,

• pristine === !dirty
• hasXErrors for existence of any error (to prevent submit)
  • xError only for whole form error
  • xErrors for errors in fields

Honestly, I think doc's comprehensibility is a big problem and barrier for me to introduce this lib for my co-worker or Japanese brothers/sisters.

To confess, until this work, I didn't understand the whole picture of relations.

What I did in this PR

• I arranged the properties into some groups
• I added some pseudo code to visualize the relations

Preview in Github

Preview in Github