XCode 4.5 downloading documentation
You can do that directly form Xcode preferences:


Open Xcode
Menu Xcode > Preferences
Go to the tab Download 
Go to the sub-tab Documentation
Select the documentation you want to re-install
Click the - icon below the documentations list for moving it to the trash.
Click install on the desired documentation


Maybe you can skip the step 6 if Xcode shows that the documentation is not installed.