n-api,doc: update doc regarding callback HandleScope/CallbackScope use

[gabrielschulhof]

Explicitly state that HandleScope and CallbackScope declarations are not required inside AsyncWorker, ThreadsafeFunction et. al. callbacks.

Re: nodejs/node-addon-examples#142

[gabrielschulhof]

It is tricky to express that opening a handle and/or callback scope is not necessary but opening a handle scope is still recommended if one is dealing with large numbers of JS values in the body of the callback.