If you have installed the new Xcode 4.2 (only a 1.6 GB file instead of the former 4GB file), you will notice that documentation does not work properly, actually people have been complaining about it and the solution was to download the documentation from inside Xcode preferences.
But if you are a team of iOS developers and want to download the documentation only once and install it on many machines running Xcode?
Here you are the steps:
1. Download the documentation from Xcode from any machine.
2. Go the download installation (See Figure Below)
3. Copy the “documentation set” files (it is actually packages with a lot of file so it is recommend to zip them).
4. Move them to the same location where you installed Xcode on the other machines
5. Restart Xcode.