units: introduce new Documentation= field and make use of it everywhere

This should help making the boot process a bit easier to explore and
understand for the administrator. The simple idea is that "systemctl
status" now shows a link to documentation alongside the other status and
decriptionary information of a service.

This patch adds the necessary fields to all our shipped units if we have
proper documentation for them.

src/core/unit.h

@@ -157,6 +157,7 @@ struct Unit {
         char **requires_mounts_for;
 
         char *description;
+        char **documentation;
 
         char *fragment_path; /* if loaded from a config file this is the primary path to it */
         usec_t fragment_mtime;