Sync docs and comments with code in hello_world example

The hello_world.cpp in tutorial uses `wait_all()` and `wait_each()`
functions, but the documentation and the comments state that `wait()`
function is used. This is not true since 50be59f8 and fca92db6.
Merge pull request #1328 from ddemidov/hello-world-fix

Sync docs and comments with code in hello_world example