systemctl: clearer error message for missing install information

Some users found it difficult to understand what systemctl was telling
them.
Instead of "install information" talk about "[Install] section", which
is more likely to ring a bell. And suggest that it is intentional, so
that users do not attempt to "correct" the unit files.

https://bugzilla.redhat.com/show_bug.cgi?id=817033

src/systemctl/systemctl.c

@@ -4209,7 +4209,7 @@ static int enable_unit(DBusConnection *bus, char **args) {
         }
 
         if (carries_install_info == 0)
-                log_warning("Warning: unit files do not carry install information. No operation executed.");
+                log_warning("The unit files have no [Install] section. They are not meant to be enabled using systemctl.");
 
 finish:
         if (m)