Remove references to boost::atomic in accumulator example

[andreasbuhr]

The documentation of the accumulator example has a paragraph about usage of boost::atomic. But usage of boost::atomic was removed from the accumulator example in 2012. It confuses the reader and should be removed.

Usage of boost::atomic was replaced by usage of hpx::components::locking_hook

[andreasbuhr]

Added some more cleanup:
There is a section about "action codes" in the documentation, but the corresponding code was removed two years ago. The definition of action codes is not necessary any more, so remove that section from the documentation.

[hkaiser]

Thanks!