[optional] Editorial fix
[cpp.scope] Properly mark macro examples as such

fixes #80
Merge pull request #141 from je4d/master

[cpp.scope] clearly mark examples.

Thanks to Jeff Snyder!
Merge pull request #136 from p12tic/master

[optional] Editorial fix to remove "optional<T>::" qualification from descriptions of optional member functions, to be consistent with the rest of the standard.
[futures.promise] Remove redundant qualification.

set_exception_at_thread_exit() has the same signature for all
specializations.
Replace NULL with null.