Improved /examples/async/hello_world.ml for newbies

As I am trying to grasp the cohttp library I am studying the hello_world.ml carefully.
I find there are a bunch of names (presumeably mostly module names) that I struggle to find the definitions of. Therefore I have added some explicit references to the code, and also I have removed the 'open' directives for clarity.
I hope you will accept my changes for the benefit of other newbies studying your cohttp library.
I also hope you will help fill in the missing references :)
ps. the reference 'j' concerns the name 'optional_with_default'.