Update Strong-remoting API docs for promises

[davidcheung]

Parent: strongloop/loopback/issues/1409
Related: strongloop/strong-docs/issues/63, strongloop-internal/scrum-loopback/issues/615

lib/remote-objects.js
RemoteObjects.after
RemoteObjects.before

lib/shared-method.js
sharedMethod.invoke

[bajtos]

@davidcheung @jannyHou this is still relevant, although possibly in a different way than was originally meant.

RemoteObjects.after
RemoteObjects.before

These two methods accept an async handler function that is called for every request. This handler function signal the completion either by accepting a callback argument, as is currently shown in our API docs; or it can return a promise.

// current example
remotes.after('dog.prototype.speak', function(ctx, next) {
  console.log('After speak!');
  next();
});

// promise version
remotes.after('dog.prototype.speak', function(ctx) {
  console.log('After speak!');
  return Promise.resolve();
});

sharedMethod.invoke

This is an internal method that intentionally does not support Promise mode, there is no need to change the current API docs (IMO).