[DOCS] Improve docs about callable property

[wgao19]

Addressing #7777, this PR hopes to improve docs around callable property.

The content of these edits credits to the discussions at niieani/typescript-vs-flowtype#55, especially insights from @omninonsense and @goodmind. Would really appreciate your inputs on this revision to the docs as well.

[goodmind]

Function statics isn't {}, or if you mean {} as unsealed object, then it is probably

https://flow.org/try/#0GYVwdgxgLglg9mABMAFASkQbwL4ChfAB0wccQA
#106

[wgao19]

Is it about function type statics? Because I read in this commit that function type statics has been changed to "an empty, inexact object type". I'm wondering if rephrasing it as follows will be more accurate and clearer:

Callable objects can be viewed equivalently as functions with static fields. Function type statics are initially {}. Flow will notice when you annotate a variable as function type and try to access its statics without annotation.

type F = () => void;

const f: F = () => {
  f.foo = 'bar'; // $ExpectError "missing in statics of function type"
}