Provide choice function or add documentation to flatAlt

As per [this](http://stackoverflow.com/a/28440998/52310) Stack Overflow answer.

It is sometimes useful to have more than 2 alternative renderings, with the widest one preferred if it fits. This is achievable using `group` and `flatAlt`, but this is not immediately apparent.

Read the flatAlt API and the linked Stack Overflow answer first. Determine whether the project should expose a choice function or document the existing group and flatAlt combination; done means the selected approach clearly supports multiple alternative renderings with the widest fitting option preferred.

Written by the indexing model from the issue text.