avocado/plugins/journal.py: make the goal of this plugin more clear

We found some users of this plugin, and we're unsure they actually use
this as intended.  Even if they don't use it as intended, it may be a
good thing, *but* we should make it clear what was the original goal
of this feature.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

avocado/plugins/journal.py

@@ -126,8 +126,11 @@ class Journal(CLI):
             return
 
         self.parser = parser
-        run_subcommand_parser.output.add_argument('--journal', action='store_true',
-                                                  help='Records test status changes')
+        help_msg = ('Records test status changes (for use with '
+                    'avocado-journal-replay and avocado-server)')
+        run_subcommand_parser.output.add_argument('--journal',
+                                                  action='store_true',
+                                                  help=help_msg)
 
     def run(self, args):
         if 'journal' in args and args.journal is True: