"Add Documentation" is disabled in Xcode 9.1 (9B55)
As KellyTheDude mentioned, renaming or moving the Xcode.app will fix the issue on OSX 10.13 (High Sierra) since /usr/libexec/xpccachectl (the preferred solution) does not seem to exist under this version of the OS. To fix quickly:
- Rename Xcode in the applications folder temporarily.
- Name it back to Xcode
UPDATE: This fix also works on macOS Mojave (10.14) and Xcode 10
UPDATE: This also fixes the Xcode Source Editor not showing up in System Preferences under Extensions (for Swiftify, etc.)
Oddly I was able to fix this by moving my Xcode.app to Applications. I originally had it nested in a subdirectory under there to version it, but alas, someone must have hardcoded a path somewhere.