openbsd: Add comment for time setting

openbsd/_openbsdcontext.py

@@ -50,6 +50,7 @@ class Context(BaseContext):
 
     # http://stackoverflow.com/questions/12081310/
     # python-module-to-change-system-date-and-time
+    # There is no 'rt' library on OpenBSD
     @staticmethod
     def _openbsd_set_time(time):
         import ctypes