How delete Documentation “iOs 4.2 Library” in XCode 4

Xcode 4:

1. Open Xcode Preferences
2. Click to highlight the docset you want to delete
3. Click the rectangle with a filled triangle icon. It’s next to the + and – icons at the bottom of the list of docsets
4. Click the hyperlink to show the docset in the finder.
5. Move or delete the docset. You will likely need administrator privileges for this.

I recommend you move them to a temporary location. Especially for older documentation, you may have to install old versions of Xcode to get them back.

UPDATE for Xcode 5:

1. Navigate to ~/Library/Developer/Shared/Documentation/DocSets/
2. Move or delete the docset. You may need administrator privileges for this.