While working on webui, I have noticed that exports from
automata-library contain for many algorithms documentation set to
string "Documentation not available.". I traced the problem back to this
project.

I do not think "Documentation not available" is itself a valid
documentation of an algorithm so I decided to replace the std::string
that is supposed to hold documentation of an overload with an
std::optional<std::string>. The semantics should now be clearer.