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.

... because most developers were copy-pasting the preambule anyway and
did not pay any attention to change it.

Also, I think that git does better job in managing that.

Explicit constructors, transform, and any_of