basic: document that people shouldn't use refcnt.h without reason

refcnt.h only exists for cases where objects are simultaneously handled
by different threads. Otherwise it should not be used. The only case
where this applies is sd_bus, really, and pretty much none of our APIs,
since we do not claim thread-safety for them.

src/basic/refcnt.h

@@ -21,7 +21,9 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-/* A type-safe atomic refcounter */
+/* A type-safe atomic refcounter.
+ *
+ * DO NOT USE THIS UNLESS YOU ACTUALLY CARE ABOUT THREAD SAFETY! */
 
 typedef struct {
         volatile unsigned _value;