I must edit my response to correct a little bit.
Related to OpenLayers there are several main documentations:
1 - A wiki like one
2 - API documentation
3 - Developer documentation
4 - Last version/trunk API documentation
5 - Last version/trunk Developer documentation
The API documentation is intended describe the stable details of the API, while the Developers documentation point to other details which are needed when developing OpenLayers applications.
The documentation above "(the API doc 2 and 4)" only includes the API Methods. Full developer/internal library documentation is also available below "(3 and 5)", but methods which are not available in the API documentation above can not be depended upon to stay the same between releases. If you use methods included in these docs, you may have to make modifications in your application code to run between minor releases of OpenLayers."
GOOD ANSWER : You should rely on the API docs of the latest release, link from point 2, to be sure your code is relative safe over the new versions.
WRONG ANSWER <>
My actual correct response is based on.
There is a similar discussion here.