[PR] Add docs on callable object types
[PR] Add docs on callable object types

Summary:
I found this functionality to be very useful, but didn't know it existed until I happened upon it in the [libdef of `React$StatelessFunctionalComponent`](https://github.com/facebook/flow/blob/a028a0fb8d1ce0ca715bd519f50916ad3cf168cc/lib/react.js#L137-L142)

This could probably belong in the object section, but I personally looked in the function section first.
Pull Request resolved: https://github.com/facebook/flow/pull/6558

Reviewed By: gabelevi

Differential Revision: D8842918

Pulled By: mrkev

fbshipit-source-id: 2a41bf2cc926cb2668ccea317ed45e4045a608b0

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