Document the strings API

[rillig]

It is difficult for a new developer to grasp why so many stringer_t types exist and what they are for. Therefore the API should either be trimmed down to the strictly necessary parts, or it should be documented by giving some typical examples of use. The documentation should be at the stringer_t data type and be cross-referenced from the other data types.

Maybe these other data types can even be made private, so that they are not part of the API.