Update "install.sh" warning to be more verbose

See https://github.com/docker/docker/issues/12395#issuecomment-99941300 for some of the context on this one.

Signed-off-by: Andrew "Tianon" Page <admwiggin@gmail.com>

hack/install.sh

@@ -49,11 +49,18 @@ do_install() {
 			;;
 	esac
 
-	if command_exists docker || command_exists lxc-docker; then
+	if command_exists docker; then
 		cat >&2 <<-'EOF'
-		Warning: "docker" or "lxc-docker" command appears to already exist.
+			Warning: the "docker" command appears to already exist on this system.
-		Please ensure that you do not already have docker installed.
+
-		You may press Ctrl+C now to abort this process and rectify this situation.
+			If you already have Docker installed, this script can cause trouble, which is
+			why we're displaying this warning and provide the opportunity to cancel the
+			installation.
+
+			If you installed the current Docker package using this script and are using it
+			again to update Docker, you can safely ignore this message.
+
+			You may press Ctrl+C now to abort this script.
 		EOF
 		( set -x; sleep 20 )
 	fi