Explicitly state that PPA is for Ubuntu only

Author: anduck

doc/build-unix.md

@@ -79,7 +79,9 @@ install necessary parts of boost:
 
         sudo apt-get install libboost-all-dev
 
-BerkeleyDB is required for the wallet. db4.8 packages are available [here](https://launchpad.net/~bitcoin/+archive/bitcoin).
+BerkeleyDB is required for the wallet.
+
+**For Ubuntu only:** db4.8 packages are available [here](https://launchpad.net/~bitcoin/+archive/bitcoin).
 You can add the repository and install using the following commands:
 
     sudo add-apt-repository ppa:bitcoin/bitcoin