Doc change to make FFI bindings easier to learn

[desmond-dsouza]

Currently the FFI docs do not show the generated Javascript, which leaves the reader guessing and makes bindings more difficult to learn. It would help if each FFI binding construct consistently had:

• bs / reason syntax including usage of the binding
• corresponding generated or equivalent javascript

(you might even put the generated js first, since that is the reader's initial intent)

Example borrowed from Glennsl's cheatsheet