This is processed by hand in CommandObjectMultiword, and is undiscoverable,
it doesn't work in all cases. Because it is a bare word, it can't really be
extended w/o introducing the possibility of collisions as well. If we did
want to do something like this we should add a --help flag to CommandObject. That
way the feature would be consistent and documented.
Differential Revision: https://reviews.llvm.org/D142067