`pretty-info` should be made public and added to the documentation

[dented42]

There is a pretty-info struct that can be used as the value for a check-info, similar to how string-info works. This is currently private and undocumented, which makes unit testing new rackunit checks difficult. It also seems like something that would be useful when making new checks.

Currently we can require it from rackunit/private/check-info, but that feels a little janky and it would be nice for it to be easier for others to discover.