Where can I find information about undocumented issues of unity and how those informations were found if there aren't documentation about then?
You can look at this post right here. I wouldn't recommend using these though due to the fact that they are undocumented and may not work anymore with an upgraded version of Unity.
The documentation for the classes you're interested in (TerrainData etc) will be complete for Unity 3, so if you have time to wait for it, that would be the best way to go really.
Things like TreeInstances and the like won't be undocumented nor unsupported anymore.